Please provide a Changelog

[Johannes Schriewer]

I am currently updating some projects that have used prior versions of this and i can't seem to find a Changelog. The major version changes so I suspect breakage. The only thing I can currently do is look at all commits or just update and hope for the best.

[Yiyu He]

https://github.com/koajs/bodyparser/blob/master/History.md

changelog is here, and it always follow semver.