Add a release date to the readme

^^ that's all ^^
Version bump: Milestone 2 Alpha 4

Now that M2A3 is released, time to change the version numbers again!
Stamps now render preview icons!

It took all day to get it to work properly, but now it does! Stamps now
render preview icons! Next I'll make a stamp name editor, and then allow
saving and opening of stamps.
Add to changelog

Add the awesome new feature to the readme changelog.