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: Ia36919df21c55bfc40497f7c174ba5919f44a394
(cherry picked from commit 3ac0351978)
ac664017e3