Add rnib functionality
- The *_pb2.py files were added that compiled by protoc
- `get_list_gnb_ids`, `get_list_enb_ids` added to the _BaseXapp
- rnib repo added as a submodule

Issue-ID: RIC-788

Signed-off-by: yc999.jang <yc999.jang@samsung.com>
Change-Id: Id5c3392592422597b939fccda902c04369566e9e
diff --git a/docs/rnib.rst b/docs/rnib.rst
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+..
+..  Copyright (c) 2021 Samsung Electronics.
+..
+..  Licensed under the Creative Commons Attribution 4.0 International
+..  Public License (the "License"); you may not use this file except
+..  in compliance with the License. You may obtain a copy of the License at
+..
+..    https://creativecommons.org/licenses/by/4.0/
+..
+..  Unless required by applicable law or agreed to in writing, documentation
+..  distributed under the License is distributed on an "AS IS" BASIS,
+..  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+..
+..  See the License for the specific language governing permissions and
+..  limitations under the License.
+..
+
+
+RNIB(Radio network information base)
+====================================
+
+RNIB information is stored in SDL by E2mgr during the E2SETUP process.
+It is saved as protobuf serialized, see [nodeb-rnib](https://gerrit.o-ran-sc.org/r/admin/repos/ric-plt/nodeb-rnib) for .proto definition.
+
+The .proto files defines the message format is located in the rnib repo.
+The rnib repo is set to submodule.
+
+**How to compile .proto files**
+
+Prerequisite
+
+* protoc (v3.6.1)
+
+.. code-block:: bash
+
+    $ PYTHON_OUT="ricxappframe/entities/rnib"
+    $ RNIB_PROTO="nodeb-rnib/entities"
+    $ protoc --python_out="${PYTHON_OUT}" \
+             --proto_path="${RNIB_PROTO}" \
+             $(find "${RNIB_PROTO}" -iname "*.proto" -printf "%f ")
+    $ sed -i -E 's/^import.*_pb2/from . \0/' ${PYTHON_OUT}/*_pb2.py
+
+**The reason why sed is needed**
+
+*To use relative imports in generated modules* : `protobuf issue
+<https://github.com/protocolbuffers/protobuf/issues/1491>`_