Add note about attributing authors of bundled code in DESC file

[mpadge]

Our "Licensing" section currently only has this:

The package needs to have a CRAN or OSI accepted license. For more explanations around licensing, refer to the R packages book.

We should just add an additional note and explicit link to https://r-pkgs.org/license.html#sec-how-to-include, which explicitly states:

You also need to include some standard metadata in Authors@R. You should use role = "cph" to declare that the author is a copyright holder, with a comment describing what they’re the author of.