Update A2ML reference

Author: Your Name

README.adoc

@@ -116,7 +116,7 @@ The Rhodium Standard Repository specification defines compliance requirements:
 
 === Document Formats
 
-* https://github.com/hyperpolymath/a2ml[a2ml] — Attested Markup Language (typed, verified documents)
+* https://github.com/hyperpolymath/a2ml[a2ml] — Attested Markup Language (typed, verified documents) (`application/vnd.a2ml`, v0.6.0 draft)
 * https://github.com/hyperpolymath/k9-svc[k9-svc] — Self-validating components (embedded contracts + deployment logic)
 * https://github.com/hyperpolymath/stateful-artefacts-for-gitforges[stateful-artefacts-for-gitforges] — Metadata-driven documentation pipeline