Add info about flash script to manual

https://github.com/analogdevicesinc/buildroot/blob/master/board/pluto/update.sh contains the necessary functions for flashing a new firmware image to the pluto.
This can be used to put a new image to the device via SSH and flash it, and removes necessity for a host devices to put the new image into the Mass-Storage of the Pluto.

Write a script which takes the position of the new firmware image and makes use of the already existing functions in the aforementioned script.