DOC: reformating readme

Signed-off-by: Raphael Defosseux <raphael.defosseux@openairinterface.org>

CHANGELOG.md

+# RELEASE NOTES: #
+
+## v1.0.0 -- May 2019 ##
+
+* First release, Able to serve a MME with basic attach, detach, release, paging procedures, default bearer only.

CONTRIBUTING.md

+# Contributing to OpenAir-CN #
+
+We want to make contributing to this project as easy and transparent as possible.
+
+Please refer to the steps described on our website: [How to contribute to OAI](https://www.openairinterface.org/?page_id=112)
+
+1. Sign and return a Contributor License Agreement to OAI team.
+2. Create an account on [GitHub](https://github.com).
+3. Provide the identifiant of this account to the OAI team (mailto:contact@openairinterface.org) so you have developer rights on this repository.
+4. The policies are described in these wiki pages: [OAI Policies](https://gitlab.eurecom.fr/oai/openairinterface5g/wikis/oai-policies-home)
+   - You can fork onto another hosting system. But we will **NOT** accept a pull request from a forked repository.
+      * This decision was made for the license reasons.
+      * The Continuous Integration will reject your pull request.
+   - All pull requests SHALL have **`develop`** branch as target branch.
+
+## Coding Styles ##
+
+We are using `clang-format` as formatting tool on the C/C++ code.
+
+At the time of writing (March 30th, 2020), we are using `clang-format` version 8.0.0 or above. By default, on a Ubuntu bionic Desktop edition, you would install version 6.0.0.
+
+So it is very likely you will have to install it manually. Again, at the time of writing, the working version we found was 9.0.0.
+
+```bash
+$ wget https://releases.llvm.org/9.0.0/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04.tar.xz
+$ mkdir clang_tmp
+$ tar xvfJ  clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04.tar.xz -C ./clang_tmp
+$ sudo cp clang_tmp/clang+llvm-9.0.0-x86_64-linux-gnu-ubuntu-18.04/bin/clang-format /usr/bin/clang-format
+$ rm -Rf clang*
+$ clang-format --version
+clang-format version 9.0.0 (tags/RELEASE_900/final)
+```
+
+How to format:
+
+```bash
+$ cd myClonedWorkspace/src
+$ clang-format -i theFilesYouWantToFormat
+```
+
+## License ##
+
+By contributing to OpenAirInterface, you agree that your contributions will be licensed under the [LICENSE](LICENSE) file in the root directory of this source tree.
+
+## Continuous Integration process ##
+
+1.  You push your modified code with the new branch onto our [official GitHub repository](https://github.com/OPENAIRINTERFACE/openair-spgwu-tiny).
+    -  Please make the name of the branch explicit and short.
+2.  You create a pull request from the [dedicated web page](https://github.com/OPENAIRINTERFACE/openair-spgwu-tiny/pulls).
+    -  The `target` (`base` in the web-page) branch **SHALL be `develop`**.
+    -  The `source` (`compare` in the web-page) branch is your branch.
+3.  Our Continuous Integration (CI) process will be triggered automatically on your proposed modified code and check the validity.
+    -  Check build
+    -  Check some formatting rules
+    -  Run a bunch of tests
+4.  If at least one of these steps fails, you will have to push corrections onto your source branch.
+    -  The step 3. will be again automatically triggered on this new commit.
+    -  Please wait that your run is finished before committing and pushing new modifications on your source branch.
+    -  That will allow fairness on the CI usage to other contributors.
+4.  When this automated process passes, one of our CI administrators will review your changes or assign a senior contributor
+  to do a peer-review.
+5.  Once the peer reviewer accepts your modification, one of our CI administrators will accept and merge your pull request
+    -  The CI will run again on the new `develop` branch commit.
+    -  The source branch WILL be deleted by one of our CI administrators.
+

README.txt → README.md

-# openair-cn-cups
-Control User Plane Separation of SPGW-C and SPGW-U
-├── http://www.openairinterface.org/?page_id=698 
+------------------------------------------------------------------------------
 
-It is distributed under OAI Public License V1.0. 
-The license information is distributed under LICENSE file in the same directory.
+                             OPENAIR-CN
+    An implementation of the Evolved Packet Core network.
     
-The OpenAirInterface CN CUPS software is composed of the following parts: 
+------------------------------------------------------------------------------
     
-openair-cn-cups
+  Openair-cn is an implementation of the 3GPP specifications concerning the 
+  Evolved Packet Core Networks, that means it contains the implementation of the
+  following network elements:
+
+  * MME,
+  * HSS,
+  * S-GW+P-GW.
+  
+  Each element implementation has its own repository: this repository (`openair-spgwu-tiny`) is meant for SPGW-U.
+
+# openair-spgwc
+
+In the Control User Plane Separation of SPGW, only the SPGW-U (User Plane) implementation is available in this repo.
+
+It is distributed under `OAI Public License V1.1`. See [OAI Website for more details](https://www.openairinterface.org/?page_id=698).
+
+The text for `OAI Public License V1.1` is also available under [LICENSE](LICENSE) file in the same directory.
+
+# Where to start
+
+  The Openair-cn SPGW-U code is written, executed, and tested on UBUNTU server bionic version.
+
+  More details on the deployment options and the supported feature set is available on this [page](docs/FEATURE_SET.md).
+
+# Collaborative work
+
+  This source code is managed through a GITHUB, a collaborative development platform
+
+  Process is explained in [CONTRIBUTING](CONTRIBUTING.md) file.
+
+# Directory structure
+
+<pre>
+openair-spgwu-tiny
 ├── build :       Build directory, contains targets and object files generated by compilation of network functions. 
-    ├── log :     Directory containing build log files.
-    ├── scripts : Directory containing scripts for building network functions
-    ├── spgw_c :  Directory containing CMakefile.txt and object files generated by compilation of SPGW-C network function. 
-    ├── spgw_u :  Directory containing CMakefile.txt and object files generated by compilation of SPGW-U network function. 
-└── ci-scripts :  Directory containing scripts for the CI process
+│   ├── log :     Directory containing build log files.
+│   ├── scripts : Directory containing scripts for building network functions
+│   └── spgw_u :  Directory containing CMakefile.txt and object files generated by compilation of SPGW-U network function. 
+├── ci-scripts :  Directory containing scripts for the CI process
 ├── etc :         Directory containing the configuration files to be deployed for each network function.
 └── src :         Source files of network functions.
     ├── common :    Common header files
@@ -22,15 +52,9 @@ openair-cn-cups
     ├── gtpv1u :    Generic GTPV1-U stack implementation
     ├── gtpv2c :    Generic GTPV2-C stack implementation
     ├── itti :      Inter task interface 
-    ├── oai_spgwc : SPGW-C main directory, contains the "main" CMakeLists.txt file.
     ├── oai_spgwu : SPGW-U main directory, contains the "main" CMakeLists.txt file.
-    ├── pfcp :      Generic PFCP stack implementation.
-    ├── pgwc :      PGW-C network fonctions procedures and contexts.
-    ├── sgwc :      SGW-C network fonctions procedures and contexts.
-    ├── spgwu :     SPGW-U network fonctions procedures and contexts.
     │   └── simpleswitch : Very Basic Switch implementation.
+    ├── pfcp :      Generic PFCP stack implementation.
     └── udp :       UDP server implementation.
+</pre>
 
-RELEASE NOTES:
-
-v1.0.0 -> First release, Able to serve a MME with basic attach, detach, release, paging procedures, default bearer only.