message: Update README for build instruction (python package)
  It seems that build_pip_pkg target is not working. The commands
  are borrowed from /.travis/python.release.sh.
diff: @@ -84,12 +84,13 @@ $ # In docker
  $ curl -OL https://github.com/bazelbuild/bazel/releases/download/0.20.0/bazel-0.20.0-installer-linux-x86_64.sh
  $ chmod +x bazel-0.20.0-installer-linux-x86_64.sh
  $ ./bazel-0.20.0-installer-linux-x86_64.sh
  +
  $ ./configure.sh
  -$ bazel build build_pip_pkg
  -$ bazel-bin/build_pip_pkg artifacts
  +$ bazel build //tensorflow_io/...
  +$ python setup.py --data bazel-bin -q bdist_wheel
  ```
  -A package file `artifacts/tensorflow_io-*.whl` will be generated after a build is successful.
  +A package file `dist/tensorflow_io-*.whl` will be generated after a build is successful.
  ### R
  