chore(docs): mention polyfills

[amir78729]

Since we are using form internals in our components and some of older Safari versions don't support this feature, we should use a polyfill and this should be mentioned in our documentation.

[mseyfayi]

I used these polyfills in my projects to work with Button and TextField from tapsioss/web-components@0.0.0-alpha-7

Chrome 64 >= (except 66):

• From core-js@3:
  • stable/array/flat
  • stable/symbol
  • stable/global-this
  • stable/promise/all-settled
• element-internals-polyfill
• @webcomponents/webcomponentsjs/webcomponents-bundle.js

Chrome 64 < & 55 >=:

• Whole of core-js@3
• @webcomponents/webcomponentsjs/custom-elements-es5-adapter.js
• @webcomponents/webcomponentsjs/webcomponents-bundle.js
• element-internals-polyfill