Update Changelog.md

Mat3 struct changed.

Changelog.md

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+# Changelog
+
+## Version 2.0.0
+
+Add collision detection with Spheres.
+
+Add some more vector/matrix math stuff.
+
+## Version 1.0.0
+
+First version of this library.
+
+Implements collision detection between AABB (Axis-Aligned-Bounding-Box) and
+AABB, AABB and Generic-Boxes, and Generic-Boxes and Generic-Boxes.
+
+Some vector/matrix math also added.

README.md

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+# 3D Collision Helpers
+
+Things to note:
+
+- `main` is the stable branch. Expect things to not break unless the major
+  version number has changed.
+- `dev` is the unstable branch. Expect force pushes and rebases on this branch.
+- Changes to be merged into `main` from `dev` will be done with an explicit
+  merge.