Document shell completion

[michidk]

We seem to support some kind of shell completion generation since we use clap_complete. The readme.md should document how to use this feature.

[Shemnei]

Good point, the documentation needs some major work. I will try to document it soonish but won't promise anything.
For now, if you are interesseted, use the punktf completions --help and punktf man --help functions for more information.