Apply HA section template to README

Change-Id: I754470cb624eb4de4d677177b59e98271412a5e2

README.md

@@ -92,48 +92,15 @@ that Cinder will only be configured with the Juju storage device.
 
 ## High availability
 
-This charm supports high availability. There are two mutually exclusive
-HA/clustering strategies:
+When more than one unit is deployed with the [hacluster][hacluster-charm]
+application the charm will bring up an HA active/active cluster.
 
-- virtual IP(s)
-- DNS
+There are two mutually exclusive high availability options: using virtual IP(s)
+or DNS. In both cases the hacluster subordinate charm is used to provide the
+Corosync and Pacemaker backend HA functionality.
 
-In both cases, the hacluster subordinate charm is required. It provides the
-corosync backend HA functionality.
-
-### virtual IP(s)
-
-To use virtual IP(s) the clustered nodes and the VIP must be on the same
-subnet. That is, the VIP must be a valid IP on the subnet for one of the node's
-interfaces and each node has an interface in said subnet. The VIP becomes a
-highly-available API endpoint.
-
-At a minimum, the configuration option `vip` must be defined. The value can
-take on space-separated values if multiple networks are in use. Optionally,
-options `vip_iface` or `vip_cidr` may be specified.
-
-### DNS
-
-DNS high availability does not require the clustered nodes to be on the same
-subnet.
-
-It does require:
-
-- an environment with MAAS 2.0 and Juju 2.0 (as minimum versions)
-- clustered nodes with static or "reserved" IP addresses registered in MAAS
-- DNS hostnames that are pre-registered in MAAS
-
-At a minimum, the configuration option `dns-ha` must be set to 'true' and at
-least one of `os-admin-hostname`, `os-internal-hostname`, or
-`os-public-hostname` must be set.
-
-The charm will throw an exception in the following circumstances:
-
-- if neither `vip` nor `dns-ha` is set and the charm has a relation added to
-  hacluster
-- if both `vip` and `dns-ha` are set
-- if `dns-ha` is set and none of `os-admin-hostname`, `os-internal-hostname`,
-  or `os-public-hostname` are set
+See [OpenStack high availability][cdg-ha-apps] in the [OpenStack Charms
+Deployment Guide][cdg] for details.
 
 ## Network spaces
 
@@ -248,3 +215,5 @@ For general charm questions refer to the [OpenStack Charm Guide][cg].
 [juju-docs-actions]: https://jaas.ai/docs/actions
 [lp-bugs-charm-cinder]: https://bugs.launchpad.net/charm-cinder/+filebug
 [lp-bug-1862392]: https://bugs.launchpad.net/charm-cinder/+bug/1862392
+[cdg-ha-apps]: https://docs.openstack.org/project-deploy-guide/charm-deployment-guide/latest/app-ha.html#ha-applications
+[hacluster-charm]: https://jaas.ai/hacluster