Better validator documentation

[joepie91]

I took the liberty of writing a clearer quick reference for the possible validators in checkit - it can currently be found here. I've organized them by 'category', and ordered them roughly in the order that people are most likely to need them, or whichever other order makes sense for that particular category.

It's missing some items from the current README and, conversely, contains some items that the current README doesn't; this is because it seems that the current README is outdated, and doesn't match the implementation (see also #84).

I'd be happy to see this merged into checkit, replacing the validator reference that's currently there... however, it seems like there are currently some unreleased changes in master, and a documentation update like this would require a patch release to make it visible on NPM, even if the library code itself hasn't changed.

I'm not sure what branch to make a PR against, hence filing this as an issue instead :) I can create a pull request if needed though, just let me know.