See the changelog for what's changed.
As for what makes this a breaking release, in #278 we decided to make the compile exercise mode log its output to stdout. Since this changes existing user behaviour in a (broadly) backwards-incompatible way, we decided to make this a major release.
The good thing is that you don't need to do anything to upgrade! 🎉
To upgrade:
git fetch
git checkout tags/3.0.0