Add a CHANGELOG for CacheMarkdownField

CHANGELOG

@@ -15,6 +15,7 @@ v 8.13.0 (unreleased)
   - Keep refs for each deployment
   - Log LDAP lookup errors and don't swallow unrelated exceptions. !6103 (Markus Koller)
   - Add more tests for calendar contribution (ClemMakesApps)
+  - Cache rendered markdown in the database, rather than Redis
   - Avoid database queries on Banzai::ReferenceParser::BaseParser for nodes without references
   - Simplify Mentionable concern instance methods
   - Fix permission for setting an issue's due date