[Keyboard] Add Cassini keyboard (#16522)

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

keyboards/protozoa/cassini/readme.md

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+# Cassini
+
+This is a standard 60% layout PCB. It supports VIA.
+
+* Keyboard Maintainer: [Yiancar](http://yiancar-designs.com/) and on [GitHub](https://github.com/yiancar)
+* Hardware Supported: A 60% keyboard with STM32F072CB
+* Hardware Availability: https://protozoa.studio 
+
+![Cassini](https://i.imgur.com/4vYgmRjh.png)
+
+## Instructions
+
+### Build
+
+Make example for this keyboard (after setting up your build environment):
+
+    make protozoa/cassini: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 protozoa/cassini:<keymap>:flash`)