Add new keyboard AT3 (#17431)

Co-authored-by: zvecr <git@zvecr.com>
Co-authored-by: Ryan <fauxpark@gmail.com>

keyboards/atset/at12/readme.md

@@ -0,0 +1,25 @@
+# ATSET AT12
+
+A special 12 key keyboard powered by QMK.
+
+* Keyboard Maintainer: [ATSET](https://github.com/anubhavd7)
+* Hardware Supported: ATmega32U4
+* Hardware Availability: Interest Check
+
+Make example for this keyboard (after setting up your build environment):
+
+    make atset/at3:default
+
+Flashing example for this keyboard:
+
+    make atset/at3:default:flash
+
+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).
+
+## Bootloader
+
+Enter the bootloader in 3 ways:
+
+* **Bootmagic reset**: Hold down the key at (0,0) in the matrix (usually the top left key or Escape) and plug in the keyboard
+* **Physical reset button**: Briefly press the button on the back of the PCB - some may have pads you must short instead
+* **Keycode in layout**: Press the key mapped to `RESET` if it is available