Add godoc comment

protoc-gen-grpc-gateway/main.go

@@ -1,3 +1,11 @@
+// Command protoc-gen-grpc-gateway is a plugin for Google protocol buffer
+// compiler to generate a reverse-proxy, which converts incoming RESTful
+// HTTP/1 requests gRPC invocation.
+// You rarely need to run this program directly. Instead, put this program
+// into your $PATH with a name "protoc-gen-grpc-gateway" and run
+//   protoc --grpc-gateway_out=output_directory path/to/input.proto
+//
+// See README.md for more details.
 package main
 
 import (