update CHANGELOG for v9.0.1 [ci skip]

Author: juergba

CHANGELOG.md

@@ -1,3 +1,13 @@
+# 9.0.1 / 2021-06-18
+
+## :nut_and_bolt: Other
+
+- #4657: Browser: add separate bundle for modern browsers (@juergba)
+
+We added a separate browser bundle `mocha-es2018.js` in javascript ES2018, as we skipped the transpilation down to ES5. This is an **experimental step towards freezing Mocha's support of IE11**.
+
+- #4653: ESM: proper version check in `hasStableEsmImplementation` (@alexander-fenster)
+
 # 9.0.0 / 2021-06-07
 
 ## :boom: Breaking Changes