explain how to subscribe to a particular launchpad tag
explain how to subscribe to a particular launchpad tag

Launchpad allows email subscriptions to all activity relating to
particular launchpad bug tags. This is a particularly handy tool for
docs liasons, so document it.

Change-Id: Ia0e7cb2e4f00af580e4e3b10952807e96bcd7a90

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