Improve documentation

[marcoieni]

From the infrastructure survey we did:

Improve the documentation for crater runs as right now the way to use the bot isn't that clear. What's the
accepted syntax, what are some example invocations of the bot. Probably also some documentation about "I
want to do X, how do I use crater for it", e.g.

• I want to test some breakage only in root crates to get a better picture of its impact
• I want to test how widespread a lint is, how do I do that

See also:

• Document crate discovery and (arbitrary) experiment creation / execution #407
• Add an example of using craterbot to run crater on regressions from the previous run #524
• craterbot: error with documented crate selection format #537
• Fix crater agents docs in forge infra-team#172