Documentation for migrating from assert_cli

[epage]

Things to cover

• Command is run at assert or earlier and not at the end of the expression
• Output predicates don't automatically trim (unless we change it, see Should assert.output("text") trim / normalize line endings? #19)
• success is not automatically asserted.

[luser]

I just did this manually and the biggest thing it took me a while to figure out was that I should be using std::process::Command, and that this crate adds additional methods via extension traits. I could also have used some more detailed documentation on how the stdout/stderr predicates work. (Even just mentioning that the argument can be a predicate and linking to the predicates crate docs would have helped!)