add sysctl startup settings

contrib/thermald/surface_pro_5/50-surface.conf

@@ -0,0 +1,2 @@
+kernel.nmi_watchdog=0
+vm.dirty_writeback_centisecs=1500

contrib/thermald/surface_pro_5/README.md

@@ -38,6 +38,7 @@ and cooling devices.
 - Install `thermal-cpu-cdev-order.xml` to `/etc/thermald/`
 - Install `thermald.service` to `/lib/systemd/system/` and overwrite the old one.
     - Maybe install to `/usr/lib/systemd/system/` too.
+- Install `50-surface.conf` to `/etc/sysctl.d/`
 - `systemctl daemon-reload`
 - `systemctl restart udev`
 - `systemctl restart thermald`