Merge pull request grpc-ecosystem#222 from sdemos/readme-update

readme: added links to additional documentation

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