README: add link to Go releases (#3526)

README.md

@@ -29,7 +29,8 @@ If you are trying to access grpc-go from within China, please see the
 
 Prerequisites
 -------------
-gRPC-Go supports the three latest releases of Go.
+gRPC-Go officially supports the
+[three latest major releases of Go](https://golang.org/doc/devel/release.html).
 
 Documentation
 -------------