Add ES Modules support status

[FluorescentHallucinogen]

Since Polymer 3.x switching to using ES Modules instead of HTML Imports, what about adding ES Modules support status?

[FluorescentHallucinogen]

@samuelli PTAL.

[samuelli]

Yep, that's a good idea. I'd probably hold off for now, since said switch is still a bit out still and support will change between now and then.

[FluorescentHallucinogen]

https://aremodulesready.com