Document supported browsers version

[constb]

Judging by the use of Object.keys and foreach I believe IE8 is not supported. Or at least requires polyfills. I'm not sure about old Opera. These still need to be supported by some projects or at least produce meaningful error messages instead of just quietly crashing. FeatureTest is an external dependency so it would be good to know what to expect from it.

[getify]

This is a great point, and I fully expect for there to be an automated test matrix eventually. I haven't gotten around to it yet, but would happily accept a PR. ;-)

Reliance on ES5 is just safe convenience at the moment, since most people who are using transpilation are targeting ES5. They are already providing proper ES5 shims for any older browsers, so the plan is to be able to just piggyback off that. I don't expect this library to provide any such shims, but we can at least document that you need to load the ES5 shims before this library if you're supporting an older browser.

Again, PR welcome if you'd like to take a stab at adding that to the README. :)