Update README with more information

README.md

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 A linear algebra and mathematics library for computer graphics.
 
+The library provides:
+
+- vectors: `Vec2`, `Vec3`, `Vec4`
+- points: `Point2`, `Point3`, `Point4`
+- square matricies: `Mat2`, `Mat3`, `Mat4`
+- a quaternion type: `Quat`
+- perspective projections
+- and more to come...?
+
+## Compilation
+
 ~~~
 rustpkg build cgmath
 ~~~
+
+## Limitations
+
+cgmath is _not_ an n-dimensional library and is aimed at computer graphics
+applications rather than general linear algebra. It only offers the 2, 3, and
+4 dimensional structures that are more than useful for most computer graphics
+applications. This design decision was made in order to simplify the
+implementation (Rust cannot paramerise over constants at compile time), and to
+make graphics-specific optimisations easier in the future. Those looking for
+n-dimensional mathematics can look to [nalgebra](https://github.com/sebcrozet/nalgebra).
+
+Currently, operators are not overloaded. This is due to limitations in Rust's
+generics that will be fixed sometime in the future.
+
+## Contributing
+
+Pull requests are most welcome, especially in the realm of performance
+enhancements and fixing any mistakes I may have made along the way. Unit tests
+and benchmarks are also required, so help on that front would be most
+appreaciated.