Add first draft of package guidelines for inclusion in the setup

[Mark-H]

As discussed on the last call, I'd work on a draft for guidelines on selecting packages to include in the setup. This is the first draft of that and I'd love to hear what people think.

[Mark-H]

Vasily commented in slack that his extras don't have (public) bug trackers because they attracted too many support questions. Instead bugs are typically reported via email or comments on new release announcements (modx.pro). Maybe the guideline for a bug tracker needs to be dropped or adjusted to allow alternative ways of handling bug reports?

The reasons I included it initially were to make sure there's a place for people to report issues they come across, and also for them to see if a project has known issues, but I can appreciate that bug trackers aren't always the best way to do that (as modmore doesn't have public trackers for premium extras either).