Merge pull request #589 from angoenka/updates_release_2.13.0

Update beam-site for release 2.13.0
tree: 72a7d7b3d1fe1ea7cd75fd2a303fd932998c035d
  1. javadoc/
  2. pydoc/
  3. .gitattributes
  4. README.md
README.md

This branch contains the generated release documentation for http://beam.apache.org.

To contribute to the website, please modify the website sources in apache/beam. See the contribution guide for details.

To add release documentation for a new release, see the release guide.