Add a bit more documentation on the electronics.

Electronics/README.md

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+## STM Original Electronics
+This directory contains the documentation of the original control electronics of the STM.  We did some modifications which are detailed below.
+
+### Overview
+![Block diagram of the STM](./STM-Functional-Overview.png)
+
+The **Control Box** schematic can be found in [`STM_Main_neu_Schaltplan_3.pdf`](./STM_Main_neu_Schaltplan_3.pdf).
+
+The **Preamplifier** schematic is [`STM_Vorverst_neu_2.pdf`](./STM_Vorverst_neu_2.pdf).
+
+### Modifications
+
+- Preamplifier
+  - *[2025-06-14]* Added a 1nF capacitor parallel to `R10` to mitigate 12kHz
+    oscillation and stabilize the amplifier.
+- Control Box
+  - *[2025-06-14]* Disconnected `R36` from `R1`/`C1` and instead connected it
+    to -15V through a separate 10k resistor.  This inverts the setpoint
+    voltage, which was necessary to make the differentiator actually calculate
+    an error difference.  Previously, it was summing the setpoint and control
+    variable...
+  - *[2025-06-14]* Cut out `R8`, the resistor parallel to the capacitor of the
+    integrator (I-controller).  This resistor was preventing the integrator
+    from stably integrating at very low frequencies.
+  - *[2025-06-15]* Added a 1µF capacitor parallel to `C7`.  This mitigates 1.47
+    kHz oscillation when the amplifier gain is high.