Update README w/changes from MediaWiki.org

This patch contains a re-write of MediaWiki's README. The current README has
beeen obsoleted by the passage of time, and its current patchwork of contents
is not especially helpful or flattering to MediaWiki. GitHub automatically
renders a repository's README below its file listings; GitHub's popularity
thus means that this file is a lot more visible than it once was and is more
likely to shape a prospective developer's first impression of MediaWiki as a
software platform.

The re-write was conducted on-wiki, at <https://www.mediawiki.org/wiki/README>,
and advertised on wikitech-l. Not much time has passed since the announcement,
but even so the updated README is already considerably better, and there's no
reason not to continue synchronizing changes from the wiki page on an ongoing
basis, so the editorial work on MediaWiki.org need not stop as a consequence of
this change being merged.

Bug: 50552
Change-Id: Iff7df2567bd90c136ef81fb8dbe7b6d4408be75f