Update CONTRIBUTING.md with contents of wiki.
Update CONTRIBUTING.md with contents of wiki.

Once this is merged we'll delete the corresponding wiki page.
lucasallan/boulder,jcjones/boulder,ibukanov/boulder,sophie-h/boulder,benileo/boulder,letsencrypt/boulder,ZCloud-Firstserver/boulder,bmw/boulder,benileo/boulder,letsencrypt/boulder,jfrazelle/boulder,benileo/boulder,jsha/boulder,benileo/boulder,jfrazelle/boulder,ibukanov/boulder,lucasallan/boulder,patf/boulder,letsencrypt/boulder,sophie-h/boulder,bmw/boulder,patf/boulder,jsha/boulder,bmw/boulder,jcjones/boulder,ZCloud-Firstserver/boulder,sophie-h/boulder,jfrazelle/boulder,jcjones/boulder,patf/boulder,lucasallan/boulder,patf/boulder,ZCloud-Firstserver/boulder,jsha/boulder,ibukanov/boulder,ZCloud-Firstserver/boulder,jfrazelle/boulder,jcjones/boulder