docs: add archive notice to README (#28279)

This repository is being archived. Added a notice directing users
to https://images.chainguard.dev/security for the latest Wolfi
security advisory data.

Signed-off-by: Dan Luhring <dluhring@chainguard.dev>

Author: luhring

README.md

@@ -1,5 +1,7 @@
 # advisories
 
+> ⚠️ **This repository has been archived.** For the latest Wolfi security advisory data, please visit <https://images.chainguard.dev/security>.
+
 Security advisory data for the Wolfi distribution.
 
 ## Introduction