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