Add Seeed XAIO RP2040 support (#538)

Fixes #537

Author: earlephilhower

README.md

@@ -29,6 +29,7 @@ See https://arduino-pico.readthedocs.io/en/latest/ along with the examples for m
 * Invector Labs Challenger RP2040 LoRa
 * Invector Labs RPICO32
 * Melopero Shake RP2040
+* Seeed XIAO RP2040
 * Solder Party RP2040 Stamp
 * SparkFun ProMicro RP2040
 * uPesy RP2040 DevKit