Add changelog for 0.21.0.
Bump version to 0.21.0.

Changes:

* Unbreak rpm packages by reverting the `%files` change from #67.
* Remove default revision. (smasset / #76)
  WARNING: This changes the default package names and metadata!
* Add support for python/pypi packages. (Mic92 / #73, smasset / #78)