Clarify what goes in `vg` vs `polliwog`

[Paul Melnikow]

Since there is a closely related library, polliwog which does a lot of computational geometry, it's useful to clarify what belongs here vs. there.

Accordingly, the idea is to clarify this library's purpose:

A vector-geometry toolbelt for dealing with 3D points and vectors

and say that anything more complicated is general computational geometry, and goes in polliwog.

In particular, transformation matrices will be scoped to polliwog.

Accordingly, the matrix functions will be removed in the next major release.

Refs:

• lace/polliwog#107
• lace/polliwog#113