docs: add archive notice to README

[adityamaru]

This repository is being archived. Adding notice to README before archiving.

---

Note

Adds an archive notice to the top of README recommending switching to the upstream-maintained action and noting no further updates.

• Docs:
  • README.md: Add an archive notice at the top:
    • Recommends switching to upstream-maintained action to leverage Blacksmith transparent caching (link provided).
    • States this action will no longer receive dependency/security updates.
    • Inserts a separator line after the notice.

^{Written by Cursor Bugbot for commit 7592765. This will update automatically on new commits. Configure here.}