docs: Rename README to README.md

docs: Rename README to README.md

The only wikitext-speficic syntax in here is the title.

Renaming this to Markdown has various benefits:

* Makes the README render properly on Gitiles, Doxygen, and Diffusion.
  The current README is not renderable in Gitiles or Doxygen.

* More familiar to new contributors in open-source.

* Consistent with other documentation content we maintain within Git,
  including all PHP and JS documentation, which via JSDoc/JSDuck/Doxygen
  have all their descriptions parsed in Markdown.

* Rich previews and syntax highlighting in many text editors
  and Git viewers.

Change-Id: I5bade76528fa3d870334d960c39db009644bb58f