add a changelog entry about SEP-1319

Author: dsp-ant

docs/specification/draft/changelog.mdx

@@ -21,6 +21,8 @@ the previous revision, [2025-06-18](/specification/2025-06-18).
 
 ## Other schema changes
 
+1. Decouple request payloads from RPC method definitions into standalone parameter schemas. ([SEP-1319](https://github.com/modelcontextprotocol/specification/issues/1319), PR [#1284](https://github.com/modelcontextprotocol/specification/pull/1284))
+
 ## Full changelog
 
 For a complete list of all changes that have been made since the last protocol revision,