update changelog for 3.0.0

there currently is no info about what is backward-incompatible between 2.x and 3.x. after some click-around on github, it seems the changes are these two:
https://github.com/BinaryMuse/toml-node/pull/53
https://github.com/BinaryMuse/toml-node/pull/53

it would be nice to update the changelog with this info.