Add favicon to the website

[gaearon]

This from https://learnredux.com looks rad:

We should do it too!

[timdorr]

@wesbos Is it cool if we use that favicon.ico?

[wesbos]

Yep - go nuts!

https://learnredux.com/images/redux-logo-fav.png

[gaearon]

Done via #1761.