Add changelog entry

Author: msimacek

CHANGELOG.md

@@ -8,6 +8,7 @@ language runtime. The main focus is on user-observable behavior of the engine.
 * Added partial support for `PYTHONHASHSEED` environment variable (also available via `HashSeed` context option), currently only affecting hashing in `pyexpat` module.
 * Implement `_csv` module.
 * Improved compatibility with PyPI packages `wheel` and `click`
+* String conversion (`__str__`) now calls `toString` for Java objects and `toDisplayString` interop message for foreign objects.
 
 ## Version 21.3.0