Update README from [[mw:README]]

Update README to version from
<https://www.mediawiki.org/w/index.php?title=README&oldid=1305801> which
was updated to include changes made in git through commit cbc3289.

The bug tracker link change from 811b2e6 was deliberately omitted from
the on wiki updates as bugzilla.wikimedia.org is now a redirect to
phabricator.wikimedia.org in the same way that bugs.mediawiki.org is and
advertising the particular bug tracking software used by the project in
the README seems unnecessary.

Change-Id: Ia73cc6c2b80e85c47a3023b7aef19a5b54aa47cf
Merge "Update README from [[mw:README]]"