Documentation/HOWTO: convert to ReST notation

This document is almost compliant with ReST notation, but some
small adjustments are needed to make it parse properly by
Sphinx (mostly, add blank lines where needed).
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>