Add float: footnote to CSS3

[TuningGuide]

In http://www.w3.org/TR/css3-gcpm/ float:footnote is described. It would be cool to see which browser support this feature.

[cvrebert]

Subfeature of #1510.

[yuchi]

Deep link is https://www.w3.org/TR/css-gcpm-3/#footnotes

[pl86]

+1