Document how to use apps from the community repo

[DouweM]

Yes, you can add apps from https://github.com/tidbyt/community as well, all you’ll need is the apps/<app>/<app>.star file and a new pixbyt.yml file as described under https://github.com/DouweM/pixbyt#31-add-an-app 2, “Option C: Create a brand-new app” (and then follow the remaining steps under “3.2. Configure the app”)

It'd be better to have the whole community repo as a submodule, and then have separate pixbyt.yml(s) for apps you want, e.g. apps/foo/pixbyt.yml with path: apps/community/foo/foo.star