Treat an optional CHANGES file similarly to the README

[wilfwilson]

This is an analogue to gap-system/ReleaseTools#109; please see there and the corresponding issue gap-system/ReleaseTools#108 for information.

[fingolfin]

CHANGES_URL is not a part of the PackageInfo.g specification, so I'd prefer not to rely on that unless we simultaneously add it there...

But instead of that, I wonder if we shouldn't instead add a more generic solution here: how about a new input for this action, say extra_files or extras or files or so, which takes a list (i.e, as a string, which spaces or commas or so as separator) of files that should be copied to gh-pages in addition to what is already being copied. That would then allow you to do what you need, but it would be more flexible and cover other cases, too.

It could be extended later on to also copy entire dirs; and to move or rename files on the fly (e.g. a/b:c/d could copy a file a/b on the default branch to c/d in gh-pages)

[wilfwilson]

I'd be up for a solution like that 🙂 I'll look into it

[wilfwilson]

I'll close this PR now in favour of a solution that follows @fingolfin's suggestion.