About rotation formula at the bottom of README.md

[ghost]

At the bottom of README.md, there is a formula for ratate function which I think has a little typo. I suppose the correct formula is:

rotate: x,y,theta ->  (x*cos(theta) - x*sin(theta), y*cos(theta) + x*sin(theta))

, where the second element of output is y*cos(theta) + x*sin(theta) rather than y*cos(theta), x*sin(theta).

Finally, thank you so much for this nice implementation of flownet !