DOC: adding changelog and contribution docs

CHANGELOG.md

+# RELEASE NOTES: #
+
+## v1.0.0 -- September 2020 ##
+
+* Initial release
+

CONTRIBUTING.md

+# Contributing to OpenAir-CN-5G #
+
+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 [Eurecom GiLab Server](https://gitlab.eurecom.fr/users/sign_in) if you do not have any.
+   - If your email domain (`@domain.com`) is not whitelisted, please contact us (mailto:contact@openairinterface.org).
+   - Eurecom GitLab does NOT accept public email domains.
+3. Provide the `username` 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 GitLab repository](https://gitlab.eurecom.fr/oai/cn5g/oai-cn5g-amf).
+    -  Please make the name of the branch explicit and short.
+2.  You create a pull request from the [dedicated web page](https://gitlab.eurecom.fr/oai/cn5g/oai-cn5g-amf/-/merge_requests).
+    -  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.
+