Consider adding a CHANGELOG.md

[demurgos]

Hi!
I have a crate using Rowan 0.10 and want to update to the latest version. I am getting a few compiler errors but should be able to sort it out.
I know that the main user of this lib is rust-analyzer, but it would still be nice to provide a CHANGELOG.md in this repo to help other people with updates.

[matklad]

We deliberately don't provide stable APIs at the moment. If updating to the new rowan is problematic, it probably best to use something else instead.

[demurgos]

Hi, thanks for your quick reply. I just finished the update so it wasn't problematic, the main change was that rowan no longer exported SmolStr.
I know that Rowan is still changing but thought a CHANGELOG.md could be helpful anyway.