tracking.el support

Wibjarm posted this in #ement.el:matrix.org:

> So it turns out to be conveniently simple to plug ement into the modeline channel tracking from circe and get my preferred setup of "tag in the modeline on new messages, desktop notification on mentions"

```el
(defun my/ement-notify-track-if-buffer (event room session)
(when (ement-notify--room-buffer-live-p event room session)
(tracking-add-buffer (map-elt (ement-room-local room) 'buffer))))
```
> It's literally just adding this to ement-notify-functions

I'm not sure what the best thing to do now is: add this to the readme as a simple example of code users can copy into their config, put it in an `ement-tracking.el` file that users can load, or put it into `ement-notify.el` in a way that avoids compile-time, load-time, and run-time warnings.