mention how to get protoc compiler in base INSTALL.md

INSTALL.md

@@ -32,6 +32,16 @@ terminal:
  $ [sudo] xcode-select --install
 ```
 
+##Protoc
+
+By default gRPC uses [protocol buffers](https://github.com/google/protobuf),
+you will need the `protoc` compiler to generate stub server and client code.
+
+If you compile from source, see below, the Makefile will automatically try
+and compile the one present in third_party if you cloned the repository
+recursively, and that it detects your system is lacking it.
+
+
 #Build from Source
 
 For developers who are interested to contribute, here is how to compile the