Add changelog

Author: xrmx

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- semantic-conventions: Bump to 1.31.0
+  ([#4471](https://github.com/open-telemetry/opentelemetry-python/pull/4471))
 - Add type annotations to context's attach & detach
   ([#4346](https://github.com/open-telemetry/opentelemetry-python/pull/4346))
 - Fix OTLP encoders missing instrumentation scope schema url and attributes