Add initial prototype.

3-mid/physics/implement/box2d/contrib/docs/testbed.md

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+# Testbed
+Once you have conquered the HelloWorld example, you should start looking
+at Box2D's testbed. The testbed is a testing framework and demo
+environment. Here are some of the features:
+- Camera with pan and zoom.
+- Mouse picking of shapes attached to dynamic bodies.
+- Extensible set of tests.
+- GUI for selecting tests, parameter tuning, and debug drawing options.
+- Pause and single step simulation.
+- Text rendering.
+
+![Box2D Testbed](images/testbed.png)
+
+The testbed has many examples of Box2D usage in the test cases and the
+framework itself. I encourage you to explore and tinker with the testbed
+as you learn Box2D.
+
+Note: the testbed is written using [GLFW](https://www.glfw.org) and
+[imgui](https://github.com/ocornut/imgui). The testbed is not part of the
+Box2D library. The Box2D library is agnostic about rendering. As shown by
+the HelloWorld example, you don't need a renderer to use Box2D.