Instruct people how they can suggest topics for the HIG

[danirabbit]

Problem

At the moment we tell people that if a topic isn't covered, they should refer to the GNOME HIG. But this doesn't really help us figure out which UX topics are missing from the HIG

Proposal

In the introduction we should probably instead ask users to use either discussions or issues to start a conversation about UX topics so that we can eventually add those topics to the HIG