Add a don't to CONTRIBUTING about merge messages

Also updated the wiki with an example message to allay any confusion
about avoiding the default merge message.

http://r-7.co/MSF-Landing-PRs

Also fix an italics where it should have been bold.
Land #4231, more explicit instructs on commit msgs

Explicitly reminds people to be wary of default (merge) messages from
Git when landing PRs on Metasploit.