Add a badge for bazel build status.

README.md

@@ -1,7 +1,7 @@
 Protocol Buffers - Google's data interchange format
 ===================================================
 
-[![Build Status](https://travis-ci.org/google/protobuf.svg?branch=master)](https://travis-ci.org/google/protobuf) [![Build status](https://ci.appveyor.com/api/projects/status/73ctee6ua4w2ruin?svg=true)](https://ci.appveyor.com/project/protobuf/protobuf) [![Build Status](https://grpc-testing.appspot.com/buildStatus/icon?job=protobuf_branch)](https://grpc-testing.appspot.com/job/protobuf_branch) [![Build Status](https://grpc-testing.appspot.com/job/protobuf_branch_32/badge/icon)](https://grpc-testing.appspot.com/job/protobuf_branch_32)
+[![Build Status](https://travis-ci.org/google/protobuf.svg?branch=master)](https://travis-ci.org/google/protobuf) [![Build status](https://ci.appveyor.com/api/projects/status/73ctee6ua4w2ruin?svg=true)](https://ci.appveyor.com/project/protobuf/protobuf) [![Build Status](https://grpc-testing.appspot.com/buildStatus/icon?job=protobuf_branch)](https://grpc-testing.appspot.com/job/protobuf_branch) [![Build Status](https://grpc-testing.appspot.com/job/protobuf_branch_32/badge/icon)](https://grpc-testing.appspot.com/job/protobuf_branch_32) [![Build Status](http://ci.bazel.io/buildStatus/icon?job=protobuf)](http://ci.bazel.io/job/protobuf/)
 
 Copyright 2008 Google Inc.