Update contributing information

Change-Id: If4d34f487b962bd83fc7cfd9540cdb36c7f0db7f
Reviewed-on: http://review.couchbase.org/39462
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>