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

README.md

@@ -1,4 +1,5 @@
-== MediaWiki ==
+MediaWiki
+===========
 
 MediaWiki is a free and open-source wiki software package written in PHP. It
 serves as the platform for Wikipedia and the other Wikimedia projects, used

README.mediawiki

@@ -1 +0,0 @@
-README