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 <[email protected]>
Co-authored-by: Peter Hutterer <[email protected]>
Signed-off-by: Jiri Kosina <[email protected]>

Author: 2 people