add readme

README.md

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+# ADC ft. PWM
+
+Code from a make-up assignment.
+
+Pin layout:
+
+5v out --> variable resistor --> PA1 **(ADC)**
+
+PE14 --> LED --> gnd **(PWM)**
+
+PA11 --> LED + Resistor --> gnd **(optional, "ON" light)**
+
+PA12 --> LED + Resistor --> gnd **(optional, "ERR" light)**
+
+---
+
+To use UART on linux:
+- install `minicom` with your package manager
+- run `./uart.sh`
+