Add 'troubleshoot' section to README
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

wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,HarriAlasi/wiki3dviewer,physikerwelt/DRMF-vagrant,HarriAlasi/wiki3dviewer,ankita-kumari/mediawiki-vagrant,stuartbman/mediawiki-vagrant,wikimedia/mediawiki-vagrant,huit/mediawiki-vagrant,HarriAlasi/wiki3dviewer,stuartbman/mediawiki-vagrant,stuartbman/mediawiki-vagrant,physikerwelt/DRMF-vagrant,huit/mediawiki-vagrant,stuartbman/mediawiki-vagrant,huit/mediawiki-vagrant,HarriAlasi/wiki3dviewer,stuartbman/mediawiki-vagrant,physikerwelt/DRMF-vagrant,wikimedia/mediawiki-vagrant,wikimedia/mediawiki-vagrant,ankita-kumari/mediawiki-vagrant,HarriAlasi/wiki3dviewer,ankita-kumari/mediawiki-vagrant,ankita-kumari/mediawiki-vagrant,physikerwelt/DRMF-vagrant,huit/mediawiki-vagrant,HarriAlasi/wiki3dviewer,ankita-kumari/mediawiki-vagrant,huit/mediawiki-vagrant,stuartbman/mediawiki-vagrant,ankita-kumari/mediawiki-vagrant,HarriAlasi/wiki3dviewer,wikimedia/mediawiki-vagrant,stuartbman/mediawiki-vagrant