Add docs for working with discard gem

[ollym]

We use discard gem, and after reading several issues/discussions in this repo we found it wasn't enough for us so we wrote a simple initializer which improves interoperability with the discard gem. The changes should be self explanatory.

[brendon]

Thanks for this :) If we go ahead and document this then it might be better to store the bulk of the information in a wiki page (I can turn this on), or in a show and tell: https://github.com/brendon/positioning/discussions/categories/show-and-tell. We can link to it from the README. That way it can be updated in the future easily and people can comment on it etc...

In saying that, if it's just a matter of having positioning being aware of the discard gem and listening to the right callbacks then perhaps we could add that as a feature instead? Would it be as simple as responding to after_discard and after_undiscard and perhaps ignoring the other callbacks if they're triggered by the discard gem?