Document that plans for stable-fix releases should be announced

It is important to announce plans for stable-fix releases to allow
contributors to cherry-pick bug fixes to the stable branch that they
want to have included into the stable-fix release.

Change-Id: I357c7011ccebcade24c8e996ee74be3ac7d451d1
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>