add changelog entry

Author: CleanCut

CHANGELOG.md

@@ -6,6 +6,7 @@
 - Fixed `CollisionPair::either_contains` to use `.contains` instead of `==`, contributed by [@etnt](https://github.com/etnt) in [#51](https://github.com/CleanCut/rusty_engine/pull/51)
 - Added `CollisionPair::either_equals_to` which uses `==`, contributed by [@etnt](https://github.com/etnt) in [#51](https://github.com/CleanCut/rusty_engine/pull/51)
 - Fixed documentation for a few fields of the `Engine` struct which were in the wrong place.
+- Improved CI caching. Builds should now generally complete in under 3 minutes.
 
 ## [5.0.6] - 2022-05-19