Add `Alternatives` section to README

[marcospb19]

I think users might want to hear about the alternatives to Ouch in the README.

1. https://github.com/mholt/archiver (closes alternative to Ouch)
2. https://www.nongnu.org/atool/ (supports everything with binaries)
3. https://github.com/magiclen/xcompress (supports a lot with binaries)

Also add an explanation of the differences between the alternatives, how they compare to ouch and what they are supposed to do.

EDIT: look at https://theunarchiver.com/command-line too