[ACTION-86] Put cache data category back to requirements

Hi Pedro,
I put the cache data category back in the requirements document. Could I 
ask you to follow up on
ACTION-85 
<https://www.w3.org/International/multilingualweb/lt/track/actions/85>: 
Provide example of cache data category.

In particular,  could include in the data category the very clear 
description of the operation and use case you gave in the call last 
week. If possible try and refer to the specific product classes listed in:
http://www.w3.org/International/multilingualweb/lt/wiki/Requirements#Product_Classes_Implementing_Requirements

But of course, if the right type of product aren't contained there, let 
me know, or feel free to add this yourself.

Let me know once you done, and I'd be happy to discuss it further 
directly in advance of the call if we need to.

many thanks,
Dave