[Keyboard] Added Koolertron AMAG23 (#11725)

keyboards/amag23/readme.md

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+# AMAG23 (Koolertron)
+
+![Koolertron AMAG23](https://i.imgur.com/DakwCxk.jpg)
+
+One Handed Macro Mechanical Keyboard, 23 Key with optional RGB backlighting.
+
+- Keyboard Maintainer: [ianmclinden](https://github.com/ianmclinden)
+- Hardware Supported: AMAG23 (ATmega32A)
+- Hardware Availability:
+    - [Newegg](https://www.newegg.com/p/32K-00H1-00001)
+    - [Amazon](https://www.amazon.com/Koolertron-Mechanical-Keyboard-One-Handed-Programmable/dp/B07RM1TBR8)
+
+Make example for this keyboard (after setting up your build environment):
+
+    make amag23:default
+
+Flashing example for this keyboard ([after setting up the bootloadHID flashing environment](https://docs.qmk.fm/#/flashing_bootloadhid))
+
+    make amag23:default:flash
+
+**Reset Key**: Hold down the *Escape* key while plugging in the keyboard.
+
+**Tips**:
+- The keyboard comes with bootloadHID installed from factory.
+- The RGB leds seem to be chained together in diagonal rows. The result is that only 7 individual colors are supported, and these are striped across the keypad.
+
+See the [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) and the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information. Brand new to QMK? Start with our [Complete Newbs Guide](https://docs.qmk.fm/#/newbs).