readme: added links to additional documentation

This commit adds links to the google cloud platform's documentation on
the custom options used by grpc-gateway to generate your http proxy.
This documentation has information about how path, query, and body
parameters are handled, as well as specific assumptions about the
arguments present for different http methods.

Fixes grpc-ecosystem#212

README.md

@@ -5,6 +5,7 @@
 grpc-gateway is a plugin of [protoc](http://github.com/google/protobuf).
 It reads [gRPC](http://github.com/grpc/grpc-common) service definition,
 and generates a reverse-proxy server which translates a RESTful JSON API into gRPC.
+This server is generated according to [custom options](https://cloud.google.com/service-management/reference/rpc/google.api#http) in your gRPC definition.
 
 It helps you to provide your APIs in both gRPC and RESTful style at the same time.
 
@@ -60,7 +61,7 @@ Make sure that your `$GOPATH/bin` is in your `$PATH`.
      rpc Echo(StringMessage) returns (StringMessage) {}
    }
    ```
-2. Add a custom option to the .proto file
+2. Add a [custom option](https://cloud.google.com/service-management/reference/rpc/google.api#http) to the .proto file
 
    your_service.proto:
    ```diff