Improved README. Added jscs and some eslint rule explanations

Added explanations for the jscs rules and started documenting the eslint rules.

I did not follow the same readme format as the jshint docs because the jscs rule set includes many rules that are best explained as a set over multiple lines and therefore don't make sense formatted as a heading. We can discuss how best to format the rules documentation in a later commit once most of the explanations for all three linters have been written.

@Raynos @kriskowal @Matt-Esch 
ESLint guidelines.