message: micro PORTING.md: prevent internal Google tools from munging
  Add a header comment to the .md file that prevents internal
  Google tools from changing the file.
diff: +<!-- mdformat off(b/169948621#comment2) -->
  +
  <!-- Define reference-style links used throughout the document -->
  [small PRs]: https://google.github.io/eng-practices/review/developer/small-cls.html
  [Micro Contributing Guidelines]: https://github.com/tensorflow/tensorflow/blob/master/tensorflow/lite/micro/CONTRIBUTING.md
  