Add note regarding package-prefix

[tekktrik]

Adds note to help prevent https://github.com/adafruit/workflows-circuitpython-libs/issues/15 in the future. It's only needed for non-Adafruit libraries. Placing the Jinja tags the way I've done prevents unnecssary whitespace from being generated so black won't automatically need to address it (I tried using %- and such but didn't get it to line up any other way).

[jposada202020]

This will need a mention in this learning guide, https://learn.adafruit.com/creating-and-sharing-a-circuitpython-library.

[tekktrik]

Tagging @kattni for reference later, but I'll submit feedback to the guide (if this gets merged).