Docs: Add generic guide

This patch adds documentation to explain concepts needed to understand
the configuration of the Block Storage (cinder) service on OpenShift.

There is general information meant for users explaining extra volumes,
placement restrictions, snippets, etc., as well as cinder specific
topics.

README.md

@@ -11,6 +11,7 @@ running RHEL for computes (and maybe swift).
 
 Some links of interest:
 
+- [Persistent Storage Guide](docs/user-guide/index.md)
 - [OpenStack Kubernetes Operators](https://github.com/openstack-k8s-operators/)
 - [Developer Docs](https://github.com/openstack-k8s-operators/dev-docs)
 - [User Docs](https://openstack-k8s-operators.github.io/openstack-operator/)