doc(licence): apply the correct license (Apache v2.0) to the repo

Signed-off-by: Raphael Defosseux <raphael.defosseux@eurecom.fr>

NOTICE.md

+`oai-cn5g-upf-vpp` repository is distributed under **Apache V2.0 License**.
+
+For more details of the license, refer to [LICENSE](LICENSE) file in the same directory.
+
+However, it also includes OAI code or third party-software indicated below.
+
+* **N/A**

README.md

@@ -18,14 +18,26 @@ At the moment, it contains the following network elements:
 
 Each has its own repository: this repository (`oai-cn5g-upf-vpp`) is meant for UPF.
 
+This `UPF` repository contains mainly patches / hacks over 2 open-source projects:
+
+- [Vector Packet Processing](https://github.com/fdio/vpp.git)
+- [User Plane Gateway (UPG) based on VPP](https://github.com/travelping/upg-vpp)
+
 # Licence info
 
-It is distributed under `OAI Public License V1.1`.
-See [OAI Website for more details](https://www.openairinterface.org/?page_id=698).
+As this repository contains mainly patches over 2 open-source projects that are
+distributed under Apache V2, it is distributed under `Apache V2.0 License`.
+
+See [Apache Website for more details](http://www.apache.org/licenses/LICENSE-2.0).
 
-The text for `OAI Public License V1.1` is also available under [LICENSE](LICENSE)
+The text for `Apache V2.0 License` is also available under [LICENSE](LICENSE)
 file at the root of this repository.
 
+Some part(s) of the repository that are decorrelated from the 2 original open-source
+projects may be under another LICENSE type.
+
+Check the [NOTICE](NOTICE.md) file for more details.
+
 # Where to start
 
 The Openair-CN-5G UPF code is written, executed, and tested on UBUNTU server bionic version.