chore: add CHANGELOG

Author: nikku

CHANGELOG.md

@@ -0,0 +1,16 @@
+# Changelog
+
+All notable changes to [diffpatch](https://github.com/bpmn-io/diffpatch) are documented here. We use [semantic versioning](http://semver.org/) for releases.
+
+## Unreleased
+
+___Note:__ Yet to be released changes appear here._
+
+* `CHORE`: make ESM only module
+* `CHORE`: move library to `lib`
+* `CHORE`: dependency bumps
+* `CHORE`: drop `formatters` main export
+
+## ...
+
+See `git log` for older updates.