[Keyboard] Add TR90 (#16838)

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

keyboards/balloondogcaps/tr90/readme.md

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+# balloondogcaps/tr90
+
+![balloondogcaps/tr90](https://i.imgur.com/GCAZotn.jpeg)
+
+A 3x3 Pro Macropad with programmer pins
+
+* Keyboard Maintainer: [Balloondog](https://instagram.com/balloondogcaps)
+* Hardware Supported: Balloondog Play Rough series case(s)
+* Hardware Availability: [Balloondog](http://store.balloondog.nl)   
+
+Make example for this keyboard (after setting up your build environment):
+
+    make balloondogcaps/tr90:default
+
+Flashing example for this keyboard:
+
+    make balloondogcaps/tr90: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 2 ways:
+* **Physical reset button**: Briefly press the button on the back of the PCB
+* **Keycode in layout**: Hold the bottom right key and then press the bottom left key to enter the bootloader