Add HACKING section to README file

Change-Id: Ic703d633b8e96d540b4875d3e8629a40e7303a93
Reviewed-on: http://review.couchbase.org/31696
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>