add a link to the shared install guide
add a link to the shared install guide

Add a link to the shared install guide to the top of the list of guides.

Change-Id: I5fee952996ffd30ee1e36541bb2112224c8a6d72
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals,openstack/openstack-manuals