docs: Add README for Unity Project

Author: jsm174

VisualPinball.Engine.PinMAME.Unity/README.md

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+# Visual Pinball Engine - PinMAME Gamelogic Engine
+*Enables PinMAME to drive VPE*
+
+## Setup
+
+Add this to your Unity project via the package manager. When using a scoped 
+registry, use `org.visualpinball.engine.pinmame`.
+
+## Usage
+
+This package adds a new Gamelogic Engine called *PinMAME*. 
+Just add the component to your table and wire up the switches, coils, lights
+and DMD.
+
+## Developer Setup
+
+This UPM package gets build and published via CI. If you want to use it locally,
+you'll need to open and compile the VS solution first, which will copy the 
+necessary binaries to the Unity package folder.
+
+## License
+
+[MIT](../LICENSE)