Document support browser versions

Please document the support browser and Node versions.

At least for the browsers, I believe this to be very limited given there isn't any employment of a browser-side polyfill here if the Fetch API is not available. I think many people would be surprised by that when coming to the "spiritual successor of request". Granted, that issue could be solved by incorporating something like #27 (using isomorphic-fetch rather than just node-fetch) if desired.