Add link for Placement API
Add link for Placement API

Adds a link for nova's placement service API to
https://developer.openstack.org/api-guide/quick-start/

Change-Id: Id8cc4901324b59bb16f43db778a3334adb39f48d

openstack/api-site,openstack/api-site,openstack/api-site,openstack/api-site