doc: add changelogs for punycode

PR-URL: #11489
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>

Author: addaleax

doc/api/punycode.md

@@ -1,4 +1,10 @@
 # Punycode
+<!-- YAML
+changes:
+  - version: v7.0.0
+    pr-url: https://github.com/nodejs/node/pull/7941
+    description: Accessing this module will now emit a deprecation warning.
+-->
 
 > Stability: 0 - Deprecated