Add 'troubleshoot' section to README

This change adds a 'troubleshoot' section to the README (with pointers to the
MediaWiki.org page for Mediawiki-Vagrant and to #mediawiki on Freenode) and
standardizes the use of imperative verbs as section headers.

Change-Id: Id97bde8291d9406a390c152dd803dff6464df83b