docs: Add section on feature requests

docs: Add section on feature requests

Small and well-defined feature requests can be very useful, but
unfortunately too many are neither small nor well-defined.

Try to improve on that by adding another README section that
explains that changes need design approval, and points to
appropriate places to have broader design discussions,
hopefully without being too off-putting.

While this is unlikely to stop all "bad" feature requests, it
at least gives us something to point to when closing requests.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3122>