Add SparkFun ProMicro RP2040 (#196)

It's still OOS, so hookup taken from their schematics.

Author: earlephilhower

README.md

@@ -15,6 +15,7 @@ See https://arduino-pico.readthedocs.io/en/latest/ along with the examples for m
 * Adafruit QTPy RP2040
 * Adafruit STEMMA Friend RP2040
 * Adafruit Trinkey RP2040 QT
+* SparkFun ProMicro RP2040
 * Generic (configurable flash, I/O pins)
 
 # Installing via Arduino Boards Manager