Adding some notes about optimizing proxies.

Optimizing proxies (like Google's [PageSpeed Service][1]) modify
resources in-flight to reduce bandwidth consumption. Mobile network
operators do the same on a much wider scale, compressing images down
to incomprehensibility in the name of efficiency. Such services will
need to be very careful about what they optimize and how when an
author chooses to validate resource integrity.

[1]: https://developers.google.com/speed/pagespeed/service