Cache files specification

Hi folks,

I've finally written the specification and updated the schema for
Mallard cache files:

http://projectmallard.org/cache/1.0/index

This is not something most of you will care about. It's something
that Mallard tools might use behind the scenes to resolve links
and extract metadata.

Nonetheless, if you enjoy reading specifications or schemas, I'd
appreciate feedback.

Thanks,
Shaun