Use 'foreign' to make older ac accept README.md

Closes #32

See merge request !7

Convert README to MD

See merge request !5

Documents can have different languages at different levels. This is
especially true with Mallard stack files, which could incorporate
some translated documents, as well as some that have not yet been
translated. Using a single global parameter completely breaks this.

We were looking for the type as an attr on the root page element,
which isn't what we have in a stack.

This still doesn't address a stack type that is an implicit first
guide and further topics, but that requires deeper surgery.

This allows us to use extra packing divs in linkdivs