Included reference to a WebP polyfill for browsers with or without WebM support.

Provided reference to a WebP polyfill that's more all-encompassing in function; this particular polyfill enables support of the image format for browsers that currently do not support WebP with or without WebM support.
Merge pull request #280 from lozandier/patch-1

Included reference to a WebP polyfill for browsers with or without WebM support