Add contributing document

Change-Id: Ic82a263bbe15900a0abb55af7ab7c7ff6d26ad37
Reviewed-on: http://review.couchbase.org/24124
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Michael Nitschinger <michael.nitschinger@couchbase.com>