Broken link on the README

[kylecoberly]

The links to the Mozilla Validator are broken.

[darylf]

This server was turned off as part of the general decommissioning of Mozilla Labs. Both David Humphrey and Jon Buckley confirmed that the site was no longer needed.

As per: https://bugzilla.mozilla.org/show_bug.cgi?id=763804#c29

I'm not sure if there's a mirror anywhere, of if we're meant to spam the main validator now (i.e., http://validator.nu)

[shakeelmohamed]

Thanks for the info @darylf