Add docs about including polyfills

The PR I promised in https://github.com/h5bp/html5-boilerplate/issues/1635.
> 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?

@FagnerMartinsBrack In general, I do agree that we should not exaggerate with links, but sometimes I think it makes sense to add them just to make everything as easy as possible (just a click away) and also to ensure that people are pointed to good resources (using a search engine doesn't always guarantee that)

Coming back to this particular case, I suggested the inclusion of the link because I've encounter quite a lot of people that didn't knew what a polyfill was.

> I suppose this assumption is the same level as CDN and conditional style sheets or conditional class names.

@FagnerMartinsBrack Yes, we should probably also include links for them. Since you brought this up, care to open a pull request?

> Btw, there is also a Modernizr reference at https://github.com/h5bp/html5-boilerplate/blob/1d0d78509dfb31ea5236a2b6430529ccfd799763/dist/doc/extend.md#miscellaneous

According to the stats, quite a lot of people visit that particular page, so I think it would be better to leave that link there as some of the users might not look at the other pages.