Save description in `description` file for local destination

[captn3m0]

Since description is already fetched from a few sources, it would be nice if it could be saved for the local destination as well.

A .git/description file or repo/description in case of bare repos is required by programs like gitweb or rgit to show a one-line description of the repository. Since this is functionally equivalent to what other source's repo.description field typically holds, we can save it as such.

The rgit demo page shows description files in use: https://git.inept.dev/

[cooperspencer]

Sure, that should be easy enough to implement. Thanks for that.