doc(readme): add seperate readme for pb/ package

pb/README.md

@@ -0,0 +1,14 @@
+# OVPM API
+
+This package holds **gRPC** definition of the *OVPM* API. 
+And there is also a REST API implementation that is generated automatically from gRPC proto files as well. REST APIs are automatically documented with swagger.
+
+|   File               |      Description                               |
+|----------------------|------------------------------------------------|
+| user.proto           | gRPC Service Definition for User Service       |
+| network.proto        | gRPC Service Definition for Network Service    |
+| vpn.proto            | gRPC Service Definition for VPN Service        |
+| network.swagger.json | Swagger Specification for the Network REST API |
+| user.swagger.json    | Swagger Specification for the User REST API    |
+| vpn.swagger.json     | Swagger Specification for the VPN REST API     |
+