Provide a way to include the generated changelog file in the release

To do a release with Gren, the workflow is:
- put a tag on a commit
- run `gren release`

After this is done, if one wants a textual `CHANGELOG.md` file, one can run `gren changelog`, which retrieves the description of the release from the repo and puts it into a file.

Then, if we want this generated `CHANGELOG.md` to be part of the repo, we add it with a commit. However, this means this file is not part of the release tarball we just made, since this new commit will come after the tag.

Could we imagine a way to be able to make a release that would include its own updated `CHANGELOG.md` file?

Empieza rastreando los puntos de entrada `gren release` y `gren changelog` y cómo se crea el tarball de la release. Se considera terminado cuando una release puede incluir su `CHANGELOG.md` generado en el tarball sin requerir un commit posterior.

Escrito por el modelo de indexación a partir del texto del issue.