message: Update readme with working bazel config flag
diff: @@ -46,7 +46,7 @@ from source.
  ```sh
  cd ${GOPATH}/src/github.com/tensorflow/tensorflow
  ./configure
  - bazel build --config opt //tensorflow:libtensorflow.so
  + bazel build -c opt //tensorflow:libtensorflow.so
  ```
  This can take a while (tens of minutes, more if also building for GPU).
  