Fix releases section in Fabric README that shows up in github.

Also point to this page from the documentation releases page,
since it includes release links to both readthedocs documentation
and github releases with release notes. Therefore we only
need to maintain the README going forward, rather than dual
maintenance in the README and docs.

Change-Id: Id49c07774b01675dac92b93b8cc5b53e10a5623f
Signed-off-by: David Enyeart <enyeart@us.ibm.com>