Add README for Adafruit Gemma M0 Band Jacket

Added tutorial code and project details for the Adafruit Gemma M0 Band Jacket.

Author: TheKitty

Gemma/GemmaM0_Band_Jacket/README.md

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+## Tutorial code for the Adafruit Learning System guide "Disco Band Camp Jacket"
+
+Guide link is https://learn.adafruit.com/Disco-Band-Camp-Jacket/
+
+Put an Adafruit Gemma M0 to work as you attach 120 Adafruit NeoPixels to the lapels of a vintage band jacket.
+Code and project by Amelia T for Adafruit Industries, 2019.
+
+See Adafruit guide & XYmap.h for pixel map for more information
+Use Adafruit Gemma M0: 60 pixels to D0, 60 pixels to D1, & button to D2 & GND
+This sketch shows mapping pixels on an irregular matrix and provides
+various examples from RGB Shades Demo Code & the FastLED demo library.
+Can easily incorporate other examples or create your own!
+
+To use:
+⦁	Press the button to cycle through available effects shown on the functions list
+⦁	Press and hold the button (>one second) to cycle through five brightness levels
+
+Special credit to RGB Shades Demo Code Copyright (c) 2015 macetech LLC
+
+This software is provided under the MIT License, Special credit to Mark Kriegsman for XY mapping code