Add documentation on cli commands (including piping)

[kratsg]

Description

cat recast.json|pyhf cls workspace.json --patch -
cat recast.json|pyhf cls <(curl -s http://physics.nyu.edu/~lh1132/workspace.json) --patch -

These need to get added to documentation somewhere.

Relevant Issues and Pull Requests

#204 introduced some of these commands for us; #205 implemented the new cls command. #197 introduced the xml2json command.

[kratsg]

It would be nice to add a video like https://asciinema.org/a/lyQ8xsxCfeSxeirSQlNSIoU4G to the docs somewhere too.

[matthewfeickert]

That video is great. I can't wait to see that in a live demo the next time we give a talk. :)