Add CHANGELOG.md

Author: ericproulx

CHANGELOG.md

@@ -7,6 +7,7 @@
 * [#2537](https://github.com/ruby-grape/grape/pull/2537): Use activesupport `try` pattern - [@ericproulx](https://github.com/ericproulx).
 * [#2536](https://github.com/ruby-grape/grape/pull/2536): Update normalize_path like Rails - [@ericproulx](https://github.com/ericproulx).
 * [#2540](https://github.com/ruby-grape/grape/pull/2540): Introduce Params builder with symbolized short name - [@ericproulx](https://github.com/ericproulx).
+* [#2550](https://github.com/ruby-grape/grape/pull/2550): Drop ActiveSupport 6.0 - [@ericproulx](https://github.com/ericproulx).
 * Your contribution here.
 
 #### Fixes