* Match the other doc files we maintain in Git under docs/ and
  for individual components, as well as all inline docs for PHP
  and JS documentation render as Markdown via JSDoc/JSDuck/Doxygen.

  This was currently the only prominent 'wikitext'-ish
  documentation file.
  Although the only wikitext-speficic syntax here was the heading.

* The current README was not shown at all in Gitiles as it requires
  the README to have a file extension, and it does not support
  '.mediawiki'.

Follows-up c0342da8.

Change-Id: I79e7e033690abe4de042969826964389ac57fcf6