Expand usage guide to illustrate namespace package usage.

[jaraco]

Do the importlib.resources documentation (or maybe packaging guides) discuss any of these use cases and best practices?

I just reviewed the docs, and they are pretty sparse about the purpose of the API. It does link to the "using" guide, and that does seem like it might be an appropriate place to publish that information.

Originally posted by @jaraco in #68 (comment)

[jaraco]

cc @warsaw

[ckp95]

yeah I went around in circles for hours today trying to solve a problem with loading resources from a namespace package. I didn't realize the fix was in here because the docs didn't mention it.

EDIT: thanks for updating the docs!