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>

README.md

@@ -51,6 +51,28 @@ you can use:
 [matrix-room]: https://matrix.to/#/#gnome-shell:gnome.org
 [discourse]: https://discourse.gnome.org
 
+## Feature requests
+
+gnome-shell is a core compoment of the GNOME desktop experience.
+As such, any changes in behavior or appearance only happen in
+accordance with the [GNOME design team][design-team].
+
+For major changes, it is best to start a discussion on [discourse]
+and reach out on the [#gnome-design matrix room][design-room],
+and only involve the issue tracker once agreement has been reached.
+
+In particular mockups must be approved by the design team to be
+considered for implementation.
+
+For enhancements that are limited in scope and well-defined,
+it is acceptable to directly open a feature request.
+
+When in doubt, it is better to ask before opening an issue.
+
+[design-team]: https://gitlab.gnome.org/Teams/Design
+[discourse]: https://discourse.gnome.org
+[design-room]: https://matrix.to/#/#gnome-design:gnome.org
+
 ## Contributing
 
 To contribute, open merge requests at https://gitlab.gnome.org/GNOME/gnome-shell.