Input: document event types and codes and their intended use

This commit adds the file Documentation/input/event-codes.txt.

Acked-by: Henrik Rydberg <rydberg@euromail.se>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
Signed-off-by: Chris Bagwell <chris@cnpbagwell.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>