Clarify usage

README.md

@@ -1,3 +1,13 @@
+This repo houses bullet 2, the physics engine used within [Xenko](https://github.com/xenko3d/xenko), its C++ / C# wrapper and build scripts for the different supported platforms.
+bullet => Official bullet repo
+libbulletc => C++ wrapper
+BulletSharp => C# wrapper around C++ wrapper
+src => build scripts
+	N.B.: Only android is up to date
+
+
+Original README content.
+
 # BulletSharp
 BulletSharp is a .NET wrapper for the [Bullet](https://pybullet.org/) physics library.