Add MNK50 (#14031)

* Add MNK50

* Update keyboards/mnk50/rules.mk

Co-authored-by: Drashna Jaelre <drashna@live.com>

* Update keyboards/mnk50/rules.mk

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

* Update keyboards/mnk50/readme.md

hihi thanks!

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

* Update keyboards/mnk50/keymaps/default/readme.md

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

Co-authored-by: yiancar <yiancar@gmail.com>
Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: Ryan <fauxpark@gmail.com>

keyboards/mnk50/readme.md

@@ -0,0 +1,32 @@
+# MNK40
+
+This is a 40% keyboard. It supports VIA.
+
+* Keyboard Maintainer: [Yiancar](http://yiancar-designs.com/) and on [GitHub](https://github.com/yiancar)
+* Hardware Supported: A 40% keyboard with STM32F072CB
+* Hardware Availability: https://www.monokei.co
+
+## Instructions
+
+### Build
+
+Make example for this keyboard (after setting up your build environment):
+
+    make mnk50:default
+
+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).
+
+### Reset
+
+- Unplug
+- Hold Escape
+- Plug In
+- Unplug
+- Release Escape
+
+### Flash
+
+- Unplug
+- Hold Escape
+- Plug In
+- Flash using QMK Toolbox or CLI (`make mnk50:<keymap>:flash`)