Add link to API doc in client module
Add link to API doc in client module

This patch adds link to Senlin API document in client module to
give developers a guide to invoke senlinclient interfaces.

Change-Id: I1671e0f0f3df15cd9667ff2b039e61d5b462e8e7

openstack/python-senlinclient,stackforge/python-senlinclient,stackforge/python-senlinclient,openstack/python-senlinclient