Add a document on how to contribute to the documentation

This is mostly a collection of thoughts for how people who want to help out
can make the docs better.  Hopefully the world will respond with a flurry
of useful patches.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>