Physics documentation?

It seems like the README doesn't have any definitions of "stiffness" or "damping". I also skimmed through the React Europe talk and didn't see any explanations.

Since this is a "physics-based" library, it would be very helpful to explain what the physics are.

There are a number of issues circling this question:
- https://github.com/chenglou/react-motion/issues/113
- https://github.com/chenglou/react-motion/issues/252
- https://github.com/chenglou/react-motion/issues/60
- https://github.com/chenglou/react-motion/issues/265#issuecomment-184697874

Some desirables:
- [ ] Explain the model: is this a linear ([Hooke's law](https://en.wikipedia.org/wiki/Hooke's_law)) spring?
- [ ] Explain the algorithm: does this do something like, say, parameterize a [step/impulse response](https://en.wikipedia.org/wiki/Step_response) function and then sample it to generate the output?

If so, some detailed information like the below image, for example, would be extremely helpful: