[Keyboard] Add handwired split tester (#5413)

* Add handwired split tester

* Update keyboards/handwired/splittest/readme.md

Co-Authored-By: nooges <nooges@users.noreply.github.com>

* Update keyboards/handwired/splittest/readme.md

Co-Authored-By: nooges <nooges@users.noreply.github.com>

keyboards/handwired/splittest/readme.md

@@ -0,0 +1,26 @@
+Split Tester
+============
+
+A two-switch tester built using two Pro Micros, mainly intended to test RGB on split keyboards. Seen here: https://www.instagram.com/p/BvCPNzynwrV/
+
+Keyboard Maintainer: [Bakingpy/nooges](https://github.com/nooges)  
+Hardware Supported: Pro Micro   
+
+Make example for this keyboard (after setting up your build environment):
+
+    make handwired/splittest:default
+
+Example of flashing this keyboard:
+
+    make handwired/splittest:default:avrdude
+
+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).
+
+Wiring
+------
+- Add switches to both Pro Micros across B3 and B6 pins
+- Add pull-up resistor to left side between VCC and F6
+- Add pull-down resistors to right side between GND and F6
+- Connect the following pins on both sides together: D0, D1, GND, VCC
+- Add I2C 4.7kOhm resistors between D0 and VCC, and D1 and VCC
+- Wire Di of RGB strip for each half to D2