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