Update protobuf readme

- Include grpc required versions
- Clarify which steps are install and which are run every time

protobuf-definitions/README.md

 
 ## Requirements
 
-* grpc 1.14.1
+* grpcio-tools 1.11.1
+* Grpc.Tools 1.14.1
-First we will install protobuf and grpcio-tools via your terminal.
+First we will follow these steps once install protobuf and grpcio-tools via your terminal.
 **Note:** If you're using Anaconda, don't forget to activate the ml-agents environment first.
 
 `pip install protobuf==3.6.0 --force`
 On Mac: `brew install protobuf`
 
 ## Running
+
+Whenever you change the fields of a custom message, you must follow the steps below to create C# and Python files corresponding to the new message.
 
 1. Open a terminal. **Note:** If you're using Anaconda, don't forget to activate the ml-agents environment first.
 2. Un-comment line 7 in `make.bat` (for Windows, use `make_for_win.bat`), and set to correct Grpc.Tools sub-directory.