Clarify some guidelines on contribution
Clarify some guidelines on contribution

This patch revises the contribution documentation for new developers.

Change-Id: If0e11caacfc514b86d7875cfa7fd2f2f74d32f74

openstack/senlin,tengqm/senlin-container,tengqm/senlin-container,openstack/senlin,stackforge/senlin,stackforge/senlin,openstack/senlin