docs: Add matrix transform.

docs/picovector.md

@@ -34,6 +34,7 @@ and then apply rotation and scale as needed.
 around point `(x, y)`.
 * `scale(scale_x, scale_t)` - apply a scale, change the size of polygons
 * `translate(x, y)` - apply a translation, change the position of polygons
+* `matrix([1.0, 0.0, 0.0, 0.0, 1.0, 0.0, 0.0, 0.0, 1.0])` - apply an arbitrary matrix transformation
 * `reset()` - reset the transform object back to the default (no transform)
 
 Internally transforms are matrix operations, so the order you apply them