These role docs aren't exactly War and Peace, but I think longer term
as we fiddle about making things generic or not and moving them
around, we'll be better off having kept ourselves to writing
*something*.
Add terse README.rst files for all existing roles, and add simple
linter check to ensure new roles get them too.
Change-Id: Ibc836310fb8a45e12c2e31f112d92509ac350413