devel guide: add words on cherry pick review process
devel guide: add words on cherry pick review process

There has been a fair amount of ambiguity, even deliberately, around
what is a valid candidate for backporting to a stable release branch of
Kubernetes.  For some the criteria is simply "critical bug fixes", but
this is also something that lacks definition.  This documentation update
attempts to better describe what is expected of and what can be expected
by somebody proposing a cherry-pick on one of our stable release branches.

Signed-off-by: Tim Pepper <4807564e5a18784afafcf20dbdb5591e844b9075@vmware.com>

kubernetes/community,pwittrock/community,kubernetes/community,kubernetes/community,pwittrock/community