HID: Add introduction about HID for non-kernel programmers

Add an introduction about HID meant for the casual programmer
that is trying either to fix his device or to understand
what is going wrong.

Signed-off-by: Marco Morandini <marco.morandini@polimi.it>
Co-authored-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>