doc: add the rules-format file (as markdown)

doc: add the rules-format file (as markdown)

Useful to have this as part of the documentation. The rendering isn't great but
at least not any worse than pure text. Markdown escapes % so explaining our use
of %S and %H would require a double % - not idea. Let's just wrap it as a code
block and done.

Includes two typo fixes too, yay.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>