Expand crate documentation

src/cgmath/lib.rs

@@ -25,7 +25,16 @@
 //! Computer graphics-centric math.
 //!
 //! This crate provides useful mathematical primitives and operations on them.
-//! It is organized into one module per primitive.
+//! It is organized into one module per primitive. The core structures are
+//! vectors and matrices. A strongly-typed interface is provided, to prevent
+//! mixing units or violating mathematical invariants.
+//!
+//! Transformations are not usually done directly on matrices, but go through
+//! transformation objects that can be converted to matrices. Rotations go
+//! through the `Basis` types, which are guaranteed to be orthogonal matrices.
+//! Despite this, one can directly create a limited rotation matrix using the
+//! `look_at`, `from_angle`, `from_euler`, and `from_axis_angle` methods.
+//! These are provided for convenience.
 
 pub use array::Array;