Add Cookbook for common patterns

Imagine you need a version of an upgrade that can be upgraded only after a certain amount of xp has been earned.
I suggest adding a feature that loads your `XpUpgrade` into both the `Skill` and `Upgrade` plugin, and merge the logic from your feature. You can also add your own plugin that depends on other plugins.

In the docs, add a `Cookbook` section that provides suggestions for these common patterns