Add docs key and point at Discourse

Add the 'docs' key and point it at a Discourse topic
previously populated with the charm's README contents.

When the new charm revision is released to the Charmhub,
this Discourse-based content will be displayed there. In
the absense of the this new key, the Charmhub's default
behaviour is to display the value of the charm's
'description' key.

Change-Id: I4b117272f68a27732809d0dddc31ac9cf69c60d9

Author: Peter Matulis

src/metadata.yaml

@@ -4,6 +4,7 @@ maintainer: OpenStack Charmers <openstack-charmers@lists.ubuntu.com>
 description: |
   Ceph is a distributed storage and network file system designed to provide
   excellent performance, reliability, and scalability.
+docs: https://discourse.charmhub.io/t/ceph-fs-docs-index/11223
 tags:
 - openstack
 - storage