more expressive README files

I think it would be nice if our `README.md` files cover more content for whoever might want a better idea of what the content of flow or the benchmarks are.

This is a very good example: https://github.com/AtsushiSakai/PythonRobotics

README files to modify:
- README.md
- examples/README.md
- flow/benchmarks/README.md