Add AtomS3R button color demo

README.md

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+# M5Stack AtomS3R Color Button
+
+A simple PlatformIO program for the M5Stack AtomS3R.
+
+Each press of the screen/button cycles through red, green, blue, yellow,
+magenta, cyan, white, and black. The selected color name is drawn in the
+center and also printed to the serial monitor at 115200 baud.
+
+## Build
+
+```powershell
+pio run
+```
+
+## Upload
+
+Connect the AtomS3R over USB, then run:
+
+```powershell
+pio run --target upload
+```
+
+If normal upload mode is not detected, hold the reset button for about two
+seconds until the internal green LED lights, release it, and retry the upload.