docs: add changelog

Author: samypr100

CHANGELOG.md

@@ -13,6 +13,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#1889](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1889))
 - Fixed union typing error not compatible with Python 3.7 introduced in `opentelemetry-util-http`, fix tests introduced by patch related to sanitize method for wsgi
   ([#1913](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1913))
+- `opentelemetry-instrumentation-httpx` Fix mixing async and non async hooks
+  ([#1920](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/1920))
 
 ### Added