Add CONTRIBUTING document

Change-Id: I2605910712d7e5a295d3cf9c723b718afe3616a4
Reviewed-on: http://review.couchbase.org/24743
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>