recommend Spectrum as <input type="color"> polyfill

Why? Because:
* It's actively maintained
* [It was good enough to get added to WebKit/Blink's dev tools](http://bgrins.github.io/spectrum/#why-devtools)
* It's MIT-licensed
* It's widely compatible (IE 6+)
* It has CI and a test suite set up
* It has an issue tracker (one of its competitors doesn't)

By recommending specific polyfill(s), you save people the time+effort of having to research+compare the various polyfill options themselves.