fix bug 1070942 - add a contribute.json file

I think it would make sense to include unconfirmed and reopened bugs in this list. We can use <a href="https://bugzilla.mozilla.org/buglist.cgi?list_id=11220418&o1=isnotempty&f1=bug_mentor&query_format=advanced&bug_status=UNCONFIRMED&bug_status=NEW&bug_status=REOPENED&product=Mozilla%20Developer%20Network">this URL</a> to do so.

I would say that we could exclude <em>unconfirmed</em> bugs, but we don't use that field consistently. There are many bugs that are marked as uncomfirmed that we plan to do regardless, as indicated by some bugs being unconfirmed but mentored.