add Matrix to Service Discovery Identities registry

[stpeter]

@guusdk sent the following note to the editor@xmpp.org list back in January:

A quick search shows that there are multiple implementations of bridges and transport between Matrix and XMPP (in various states of completeness). This points at a real-world demand for interoperability between the two.

To facilitate this, the Service Discovery Identities registry, as published on https://xmpp.org/registrar/disco-categories.html, should include a definition for Matrix in the category section.

This seems reasonable and I'll assign it to myself.