Auto merge of #6884 - ehuss:changelog, r=alexcrichton

Add a changelog.

I plan to periodically update this for each release.

I went back about 6 months just to fill it up a little.

[Rendered](https://github.com/rust-lang/cargo/blob/2cbb701b9205213a2864edf2daa314e7ad225f4c/CHANGELOG.md)

Author: bors