Update CHANGELOG.md
Update CHANGELOG.md

I reviewed all past PRs and added notes for significant changes. I did not address changes to the component interface yet (any PRs labeled New MCI). I also started to run out of steam and did not add items for a few others, as noted.
opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core