Adds a tutorial of sorts for working with cgmath's vectors.

There are several runnable code examples that should compile when
using `make doctest`.

Signed-off-by: Aaron Jacobs <atheriel@gmail.com>
Merge pull request #106 from atheriel/vecdocs

Add a tutorial of sorts for working with cgmath's vectors.
Fixes Matrix2::from_angle invocation and adds a test for the feature.

Signed-off-by: Aaron Jacobs <atheriel@gmail.com>
Merge pull request #109 from atheriel/rot-fix

Fix `Matrix2::from_angle()` invocation and add a test for the feature.