add grpc example (#1401)

use grpc helloworld example.

examples/grpc/README.md

@@ -0,0 +1,19 @@
+## How to run this example
+
+1. run grpc server
+
+```sh
+$ go run grpc/server.go
+```
+
+2. run gin server
+
+```sh
+$ go run gin/main.go
+```
+
+3. use curl command to test it
+
+```sh
+$ curl -v 'http://localhost:8052/rest/n/thinkerou'
+```