Add docs and example for nomodule

[probins]

As there seems to be general agreement on a nomodule attribute for <script> whatwg/html#2261, I'd suggest adding an example which uses native ES loader if supported, with a nomodule script loading the polyfill with System.register modules if not. This scenario could also usefully be documented both in this repo and in the main es-module-loader one.

Should I put together a PR?

[guybedford]

I'm not going to advocate a workflow that isn't yet proven to be useful.