Input: document device properties

Add a section which defines the input device properties and provides
guidelines on how to use them.

Signed-off-by: Henrik Rydberg <rydberg@euromail.se>
Reviewed-by: Chase Douglas <chase.douglas@canonical.com>
Reviewed-by: Jussi Pakkanen <jussi.pakkanen@canonical.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>