Add docs about including polyfills

The PR I promised in https://github.com/h5bp/html5-boilerplate/issues/1635.
I was going to make that suggestion when this PR was submitted, but is it reasonable to polute the document by link the meaning of "polyfill", considering this is such a basic concept easly found on Google?

I suppose this assumption is the same level as [CDN](https://github.com/h5bp/html5-boilerplate/blob/1d0d78509dfb31ea5236a2b6430529ccfd799763/dist/doc/faq.md#why-dont-you-automatically-load-the-latest-version-of-jquery-from-the-google-cdn) and [conditional style sheets or conditional class names](https://github.com/h5bp/html5-boilerplate/blob/1d0d78509dfb31ea5236a2b6430529ccfd799763/dist/doc/css.md#the-css). In those cases there is no link to explain the meaning, indicating there are some basic assumptions about the reader.

Btw, there is also a Modernizr reference at https://github.com/h5bp/html5-boilerplate/blob/1d0d78509dfb31ea5236a2b6430529ccfd799763/dist/doc/extend.md#miscellaneous