README: update developer documentation link

The old URL doesn't work anymore; point it at the updated path.

Signed-off-by: Daniel Verkamp <daniel@drv.nu>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20260409065537.3440067-1-daniel@drv.nu>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>