Adding Makefile for easy building.

Makefile

+UNITY=/Applications/Unity/Hub/Editor/2019.3.3f1/Unity.app/Contents/MacOS/Unity
+UNITY_BATCH:=${UNITY} -batchmode -quit -noUpm -projectPath $(CURDIR)
+
+.PHONY: build
+build: Build/physicssimulator.app
+
+Build/physicssimulator.app: | Build
+	-${UNITY_BATCH} -buildOSX64Player $@
+
+Build:
+    @-mkdir -p $@
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