cross-platform backup tool Call to action for shell completion

Hi,

with the last pull request, it became much easier to write one's own completion script, because rdiff-backup itself is doing the hard work (the bash completion script went from 95 to 20 lines of code).

This makes maintenance of additional shell completions much easier to write and maintain. Hence this call to all users of alternative shells (Csh, fish, ksh, zsh, etc) to write and provide their own completion script to the community.

Details on how this works are available at https://github.com/rdiff-backup/rdiff-backup/blob/master/docs/arch/completion.adoc and of course you can take the bash completion script as example. If something is unclear, let me know.

Thanks, Eric