Add new directory for shield PCB

quad/Shield PCB/README.md

+# Shield PCB
+This PCB is designed to simlpify the assembly and re-assemmbly of the quads, as well as the reliability.
+It does so by providing dedicated connections to each external device on separate, latching connectors.
+
+# Errata/Easy Mistakes
+## Version 2
+ - UART0 lines (to WiFi) are flipped, can be fixed in constraints file.
+ - RC controller PWM lines are reconfigurable in software and constraints,
+ and depend on how the RC receiver is wired, so double check that you get good results if you change those.
+ - Motor PWM lines are reconfigurable in software, and have to match the controller, so be careful with those as well.
+ - The IMU has to be mounted with the label text down (the lables for the pins) and the mounting holes facing towards the PMOD test points
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