Add a README section about supported versions

[henrymercer]

This information is already included in the changelog, but let's surface it in the README to so that it's more visible.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[jsoref]

Thanks. This text is very helpful.

It's probably worth mentioning that workflows running outside GHES on v2 will trigger node16 depreciation warnings on each and every run.

It's a bit surprising. (I have a customer that filed a ticket asking me to release an upgrade so that the warning goes away. - thankfully I had already been working on fixing it because I also find the warning very annoying, but it took me quite a bit to develop and validate a fix for one of the other breaking action updates.)

[aeisenberg]

Thanks for writing this up.

[henrymercer]

I think I'll merge this as is, since we produce our own warning as well about upgrading from v2 to v3 (when the user's running on a GitHub instance that supports Node 20 Actions).