Document changes between releases

I just noticed that [version 0.0.10](https://github.com/ckan/datapusher/releases/tag/0.0.10) has been released :tada: (an announcement on the ckan-dev mailing list would have been awesome!).

The decision whether I should update immediately or whether it's a nice-to-have upgrade depends on the changes since 0.0.9. However, these don't seem to be documented anywhere.

So, please [start providing a changelog](http://keepachangelog.com/en/0.3.0/). While we're at it, [semantic versioning](http://semver.org/) would be really cool, too.