Update Fediverse app criteria (added link to client watchlist and explanation)

Fediverse-app-criteria.md

@@ -10,4 +10,8 @@
   * be followed
   * post a reply
   * receive a reply
-  * repost
+  * repost
+
+#### Tracking fediverse app development
+
+✒ A general [watchlist of fediverse client apps](https://gitlab.com/fediverse/fediverse.gitlab.io/wikis/watchlist-for-client-apps) is being maintained on this wiki. Please add as much information as you can about each app, but please check your facts, and try to include links to primary sources.