unity: Add .gitignore and README.

Author: freezy

VisualPinball.Engine.Mpf.Unity/.gitignore

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+# don't commit DLLs - the VS solution creates them.
+Plugins/**/*.dll

VisualPinball.Engine.Mpf.Unity/README.md

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+# Visual Pinball Engine - MPF Gamelogic Engine
+*Enables the Mission Pinball Framework to drive VPE*
+
+## Setup
+
+Add this to your Unity project via the package manager. When using a scoped 
+registry, use `org.visualpinball.engine.missionpinball`.
+
+## Usage
+
+This package adds a new Gamelogic Engine called *Mission Pinball Framework*. 
+Just add the component to your table and wire up the switches, coils and 
+lights.
+
+## 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)