Add the RP2040 Stamp board (#407)

Author: arturo182

README.md

@@ -26,6 +26,7 @@ See https://arduino-pico.readthedocs.io/en/latest/ along with the examples for m
 * Invector Labs Challenger RP2040 LTE
 * Invector Labs RPICO32
 * Melopero Shake RP2040
+* Solder Party RP2040 Stamp
 * SparkFun ProMicro RP2040
 * uPesy RP2040 DevKit
 * WIZnet W5100S-EVB-Pico