docs: Add entry to changelog

CHANGELOG.md

@@ -9,6 +9,16 @@ and adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0).
 
 ### Added
 
+- Instrumentator now works without FastAPI. This is possible because every
+  FastAPI app is also a Starlette app (but not the other way around). Or to be
+  more specific: FastAPI uses Starlette for things like routing and middleware
+  this package relies on. The change is backwards compatible, even type checkers
+  like mypy should continue working. Thanks to
+  [@mvanderlee](https://github.com/mvanderlee) for proposing this in
+  [#280](https://github.com/trallnag/prometheus-fastapi-instrumentator/issues/280)
+  and implementing it in
+  [#288](https://github.com/trallnag/prometheus-fastapi-instrumentator/pull/288).
+
 - Relaxed type of `get_route_name` argument to `HTTPConnection`. This allows
   developers to use the `get_route_name` function for getting the name of
   websocket routes as well. Thanks to [@pajowu](https://github.com/pajowu) for