add short readme

README.md

+# HighFinesse PSU control PCB
+This repository contains the design files for a control board to program
+the DAC in the digital control option of the HighFinesse power supplies.
+
+The board has an ethernet input for communication with the microcontroller
+and another RJ45 output for connection to the power supply.
+Synchronisation with the experiment must be performed using trigger pulses
+sent to the BNC input.
+
+This board is completely untested as the main MCU chip was not deliverable
+in time due to the global chip shortage. It will require minor
+modifications to the [firmware][1], which is currently expecting a
+modified version of the CamDDS board.
+
+ [1]: https://gitlab.physik.uni-muenchen.de/cs/highfinesse-psu-spi-controller-firmware