Add CHANGELOG.md

[demurgos]

I'm in the process of updating a crate that depended on language-tags 0.2 to the latest version (0.3.2). The lack of CHANGELOG.md file in the repo with a summary of changes makes it harder. I'll deal with my issue by checking the commits directly, but I still recommend to keep a changelog so it's easier to track the evolution of this lib.

You don't have to backfill entries for previous versions, but having it for future releases would be great.