Clarify features repo issue is required
Clarify features repo issue is required

A recent exception request linked to a k/k issue covering the feature
and triggered discussion on whether that is sufficient.  This change
clarifies that a feature issue in the feature repo is expected.

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

kubernetes/enhancements,kubernetes/enhancements,kubernetes/enhancements