Add information on how to contribute

The CONTRIBUTING.ms describes how to setup your environment
and how to upload changes to Gerrit.

Change-Id: Ib28cb1fea8019cfbfc4ba7dc330fbe476b11be90
Reviewed-on: http://review.couchbase.org/25758
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>