Documentation: Write some docu

* Safety notice
* HW modifications
* Links to PCB repo

doc/source/hardware/modifications.rst

@@ -0,0 +1,15 @@
+.. _hardware_modifications:
+
+Hardware Modifications
+======================
+
+Analog Frontend
+---------------
+
+.. note::
+	Solder a ``100 nF`` capacitor parallel to ``R315`` in order to implement a low pass characteristic of the difference amplifier in the :ref:`analog frontend<hw_analog_fe>`. This massively increases EMI performance which prevents the :ref:`ADC Watchdog<firmware_meas_adc_watchdog>` from triggering.
+
+Power Supply
+------------
+.. note::
+	Replace transformer ``BV-EI-303-2010`` with ``BV-EI-303-2050``, which has the same footprint but a little more power and does not heat that much.