Address how to contribute to existing modules

W/o this patch, it is unclear how to contribute
to existing fuel library modules. In order to fix this,
fuel dev docs should be updated as appropriate.

Closes-bug: #1382432

Change-Id: I3d152c9b09297f3ae01e54f364723ef904f9f7c5
Signed-off-by: Bogdan Dobrelya <bdobrelia@mirantis.com>
Merge "Address how to contribute to existing modules"