The PhysicsOptions constructor now properly constructs the expected type.

Before, client code was accessing a union member that hadn't been set to
at all, which I believe is undefined behavior.
The Paddle now has a *max* velocity, albeit ignored.
The max_velocity is used in the physics code.

Although it is factored in, the value is not checked. That means that if
the value is too large a paddle could very well go through a ball, etc.
Updated README.md.

Most everything has sort of been clarified or tweaked. I also added a
Roadmap, which is cool.