Placeholders supplementary, warn against polyfill

[zachleat]

Per discussion included in #158

[zachleat]

Do you mean recommend use or not use? I'm getting mixed signals here.

[nimbupani]

I mean recommend use :/ I mean if they are supplementary there is no harm 'using' them no? Our usage of the word 'use' is merely an indication that there is no threat to breaking across all browsers that do not implement this feature when used.

[addyosmani]

@zachleat if you get a chance could you look at @nimbupani's feedback above and adjust accordingly?

[zachleat]

Done!

[addyosmani]

Thanks!