README: Link to the new github.io/hid-replay-docs documentation

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
define _GNU_SOURCE to silence compiler warning

hid-recorder.c: In function ‘scan_devices’:
hid-recorder.c:142:2: warning: implicit declaration of function ‘asprintf’
[-Wimplicit-function-declaration]

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>