Mentioning NonGNU ELPA

[phikal]

Hey, this is just to let you know that htmlize was added to the NonGNU ELPA package repository: https://elpa.nongnu.org/nongnu/htmlize.html

This shouldn't affect packaging, the main difference is that while MELPA uses tagged commits to identify new (stable) versions, NonGNU ELPA uses those commits that increment the version tag in the package header.

As the repository will be active by default from Emacs 28 onwards, you might be interested in adding a note that it can be installed without any further configuration.