Add a changelog

[gilch]

Hissp is in a feature freeze for the time being. Hebigo needs to catch up, and I need to dogfood it more before I'll feel confident where to take it next. I will continue to fix bugs and consider issues as they come up.

I'll be maintaining a changelog for any new versions released, and realistically, that means every PR needs to keep it up to date. Probably should have done this earlier. I understood the changes for each version pretty well before, but the time gap between 0.4.0 and 0.5.0 means I don't remember well enough how 0.4.0 worked differently. Both versions do have pretty thorough docs, but for anyone following only the released versions, instead of latest, a changelog will be helpful.