Added Thunderbird Address Book sync instructions

Thunderbird instructions from https://github.com/owncloud/contacts/issues/87#issuecomment-18048353 incorporated into the manual.

Please check final result.  I have zero experience with reST Markup language and no way to verify results on this end.

I don't know how to delete a "commit" below.  The one I would like committed is  <b>cb9b53e</b>.  The other one can be rejected/deleted.