Update DevStack README
Update DevStack README

Reuse the content of doc/source/install/devstack.rst

Change-Id: Iaa3921fd295c8f43d9b6fdf191d44ab6b2448c9e

priya-pp/Tacker,trozet/tacker,zeinsteinz/tacker,stackforge/tacker,openstack/tacker,zeinsteinz/tacker,priya-pp/Tacker,openstack/tacker,openstack/tacker,stackforge/tacker,trozet/tacker