better polyfills with built-in support

[AndyOGo]

Unfortunately the custom element polyfill provided by @webcomponents does not support extended built-in custom elements, has this known limitations and is slow.

The most bullet proof and battle tested polyfills I found are from @WebReflection. Please consider to at least mention them, if not just replacing by those:

• document-register-element (supports V1 despites it's name) has fantastic browser support and a public test page, please note the constructor caveat.
• built-in-element again fantastic browser support and live test page, please note the constructor caveat.

[AndyOGo]

@shawnbot Any free time available to look into this?

[shawnbot]

Hi, I'm actually a huge fan of @WebReflection and have used his v0 polyfill extensively! However, to more closely align with what my teammates at GitHub are doing, I decided to slim down the polyfills section and reduce confusion around which polyfill(s) folks should use.

But! I recognize that there are some issues with the @webcomponents implementation and definitely think it's worth including links to Andrea's work here. I would gladly welcome a PR to add them back. ❤️

[AndyOGo]

@shawnbot I created a PR, please let me know if you agree with the wording