Update README.md

Added a section on protocol.

README.md

@@ -16,3 +16,16 @@ distributed applications and services. The libraries in this repository
 provide a concrete implementation of the gRPC protocol, layered over HTTP/2.
 These libraries enable communication between clients and servers using any
 combination of the supported languages. 
+
+Developers using gRPC typically start with the description of an RPC service
+(a collection of methods), and generate client and server side interfaces
+which they use on the client-side and implement on the server side.
+
+Protocol
+--------
+
+The gRPC protocol specifies the abstract requirements for communication between
+clients and servers. A concrete embedding over HTTP/2 completes the picture by
+fleshing out the details of each of the required operations.
+
+