readme: add travis and godoc buttons

README.md

 Go generated proto packages
 ===========================
 
+[![Build Status](https://travis-ci.org/google/go-genproto.svg?branch=master)](https://travis-ci.org/google/go-genproto)
+[![GoDoc](https://godoc.org/google.golang.org/genproto?status.svg)](https://godoc.org/google.golang.org/genproto)
+
 > **IMPORTANT** This repository is currently experimental. The structure
 > of the contained packages is subject to change. Please see the original
 > source repositories (listed below) to find out the status of the each