Add a changelog mention

CHANGELOG.md

@@ -26,10 +26,12 @@ The following components are considered to be internal and they are excluded fro
 [#231]: https://github.com/rust-marker/marker/pull/231
 [#232]: https://github.com/rust-marker/marker/pull/232
 [#239]: https://github.com/rust-marker/marker/pull/239
+[#259]: https://github.com/rust-marker/marker/pull/259
 
 ### Added
 - [#232]: Add scope config for visitors and `for_each_expr` to `marker_utils`
 - [#239]: GitHub releases now provide precompiled binaries of `cargo-marker` and `marker_rustc_driver`.
+- [#259]: Introduced a GitHub Action for installing and running Marker
 - [#231]: Significantly improved error handling
 
 ### Internal