System tray works on Gnome

[Hi, I am that Gnome user,](https://github.com/ActivityWatch/docs/issues/26#issuecomment-538463672)

I was trying out ActivityWatch yesterday for the first time. I noticed that in the documentation you stated that the system tray icon doesn't work but it appeared to. I did some research and it was because when I installed [Steam](https://store.steampowered.com/) (that has it's own system tray menu) it had installed [AppIndicator and KStatusNotifierItem Support](https://extensions.gnome.org/extension/615/appindicator-support/), which let all of my system tray applications work again. Everything in the system tray works as expected (after a reboot) so I would like to see this in the documentation to help other people out.