Update README.md

Author: yugui

README.md

@@ -21,7 +21,7 @@ That's what grpc-gateway helps you to do. You just need to implement your gRPC s
 Then the reverse-proxy generated by grpc-gateway serves RESTful API on top of the gRPC service.
 
 ## Installation
-First you need to install ProtocolBuffers 3.0 or later.
+First you need to install ProtocolBuffers 3.0.0-beta-3 or later.
 
 ```sh
 mkdir tmp