Update README.md to include browser support and polyfills docs (#4601)
Update README.md to include browser support and polyfills docs (#4601)

Fixes #4572

This PR intends to address https://github.com/zeit/next.js/issues/4572 around documenting browser support and what needs polyfills.

There are many outstanding issues on the tracker regarding browser support and polyfills. This may alleviate some of them.
JeromeFitz/next.js,flybayer/next.js,azukaru/next.js,zeit/next.js,flybayer/next.js,JeromeFitz/next.js,JeromeFitz/next.js,zeit/next.js,BlancheXu/test,azukaru/next.js,BlancheXu/test,BlancheXu/test,azukaru/next.js,azukaru/next.js,flybayer/next.js,JeromeFitz/next.js,zeit/next.js,flybayer/next.js