Create readme.md for prop multi mesh

FuncGodot Entities/GD Script/prop_multi_mesh/readme.md

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+PropMultiMesh is a custom Godot class that generates mesh instances across the surface of targeted geometry. Surface targets can be picked by either proximity or matching the target property with an entity's globalname property.
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+You can specify a resource path to a model or you can target another solid entity by globalname property to copy the mesh from to use as your multimesh.
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+Does not have any dependencies on other classes, so should be able to just drag and drop the scripts and resource into your project.