Add Hackage-supported `changelog.md` file

The contents has been ripped from the release notes. I've used markdown
as this may allow for automatic conversion to docbook XML in the future,
should we choose to maintain the `base` changelog in a more human-friendly
markup format and closer to the `base`-source tree.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>