Add JS style guide?

[melanierichards]

PR #363 adds a CSS style guide. Some of that will be condensed when we hook up the CSS linter, but there's still some styles that I linter can't really cover, so we need a linter.

We likely need the same for JS. At the very least, we should have a link to the linting rules, and a link to the ES Lint repo page that explains what these rules mean. There's probably too many of them to have a human-readable list inline :/