[#2] Added Licensing.

I decided on the Apache-2.0 license and have stamped all the files to
now point to that license.

examples/vector.rs

@@ -1,3 +1,6 @@
+// SPDX-License-Identifier: Apache-2.0
+// Sealed with Magistamp.
+
 use sigils::vector::{EuclideanVector, Vector2, Vector3, Vector4};