JS Guide: Add caniuse.com links as reference

[gerardo-rodriguez]

Interesting. On this note, is there any thought of adding comments about native support (thinking browsers), so that we don't end up relying on Babel or polyfills to use what should be more performant, future-friendly methods, but are actually being transpiled to something less performant, at least for some clients.

I realize this is a moving target and also very much a case-by-case thing, but even links to caniuse might be helpful.

Originally posted by @Josh68 in https://github.com/cloudfour/guides/pull/35