Update README.md

Point users to the proto definition instead of Service Management API documentation. Latter is generated from former, and is more likely to be out of date.

Rephrase "custom options" to "google.api.http" annotation to be more straight forward.

README.md

@@ -5,7 +5,9 @@
 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.
+This server is generated according to the
+[`google.api.http`](https://github.com/googleapis/googleapis/blob/master/google/api/http.proto#L46)
+annotation in your gRPC service definition.
 
 It helps you to provide your APIs in both gRPC and RESTful style at the same time.
 
@@ -20,7 +22,7 @@ However, you might still want to provide a traditional RESTful API as well. Reas
 This project aims to provide that HTTP+JSON interface to your gRPC service. A small amount of configuration in your service to attach HTTP semantics is all that's needed to generate a reverse-proxy with this library.
 
 ## Installation
-First you need to install ProtocolBuffers 3.0.0-beta-3 or later.
+First you need to install ProtocolBuffers 3.0.0 or later.
 
 ```sh
 mkdir tmp
@@ -59,7 +61,7 @@ Make sure that your `$GOPATH/bin` is in your `$PATH`.
      rpc Echo(StringMessage) returns (StringMessage) {}
    }
    ```
-2. Add a [custom option](https://cloud.google.com/service-management/reference/rpc/google.api#http) to the .proto file
+2. Add a [`google.api.http`](https://github.com/googleapis/googleapis/blob/master/google/api/http.proto#L46) to your .proto file
 
    your_service.proto:
    ```diff
@@ -83,7 +85,7 @@ Make sure that your `$GOPATH/bin` is in your `$PATH`.
     }
    ```
 
-   If you do not want to modify the proto file for use with grpc-gateway you can alternatively use an external [gRPC API Configuration](https://cloud.google.com/endpoints/docs/grpc/grpc-service-config) file. [Check our documentation](https://grpc-ecosystem.github.io/grpc-gateway/docs/grpcapiconfiguration.html) for more information.
+   If you do not want to modify the proto file for use with grpc-gateway you can alternatively use an external [gRPC Service Configuration](https://cloud.google.com/endpoints/docs/grpc/grpc-service-config) file. [Check our documentation](https://grpc-ecosystem.github.io/grpc-gateway/docs/grpcapiconfiguration.html) for more information.
 
 3. Generate gRPC stub