Add unreleased section to CHANGELOG post 0.39.0 release
Add unreleased section to CHANGELOG post 0.39.0 release

sgebbie/ponyc,jemc/ponyc,dipinhora/ponyc,sgebbie/ponyc,dipinhora/ponyc,sgebbie/ponyc,ponylang/ponyc,dipinhora/ponyc,jemc/ponyc,sgebbie/ponyc,jemc/ponyc,sgebbie/ponyc,ponylang/ponyc,ponylang/ponyc