message: Fix incorrect link in `dockerfiles/README.md`
  This fix fixes incorrect link in `dockerfiles/README.md`.
diff: This directory houses TensorFlow's Dockerfiles. **DO NOT EDIT THE DOCKERFILES
  MANUALLY!** They are maintained by `assembler.py`, which builds Dockerfiles from
  -the files in `partials/` and the rules in `spec.yml`. See [the Maintaining
  -section](#maintaining) for more information.
  +the files in `partials/` and the rules in `spec.yml`. See [the Contributing
  +section](#contributing) for more information.
  ## Building
  