Update docs based on PR #99 changes

Author: mickaelseznec

README.md

@@ -145,11 +145,10 @@ rotate: x,y,theta ->  (x*cos(theta)-x*sin(theta), y*cos(theta), x*sin(theta))
 ```
 
 #### Rotation applied on img2
-We consider the angle `theta` small enough to linearize `cos(theta)` to 1 and `sin(theta)` to `theta` .
+Let us consider a rotation by the angle `theta` from the image center.
 
-x flow map ( `flow[:,:,0]` ) will get a shift proportional to distance from center horizontal axis `j-h/2`
-
-y flow map ( `flow[:,:,1]` ) will get a shift proportional to distance from center vertical axis `i-w/2`
+We must tranform each flow vector based on the coordinates where it lands. On each coordinate `(i, j)`, we have:
 ```
-\for_all i,j flow[i,j] += theta*(j-h/2), theta*(i-w/2)
+flow[i, j, 0] += (cos(theta) - 1) * (i  - w/2 + flow[i, j, 0]) +    sin(theta)    * (j - h/2 + flow[flow[i, j, 1])
+flow[i, j, 1] +=   -sin(theta)    * (i  - w/2 + flow[i, j, 0]) + (cos(theta) - 1) * (j - h/2 + flow[flow[i, j, 1])
 ```

flow_transforms.py

@@ -167,7 +167,7 @@ class RandomRotate(object):
     angle: max angle of the rotation
     interpolation order: Default: 2 (bilinear)
     reshape: Default: false. If set to true, image size will be set to keep every pixel in the image.
-    diff_angle: Default: 0. Must stay less than 10 degrees, or linear approximation of flowmap will be off.
+    diff_angle: Default: 0.
     """
 
     def __init__(self, angle, diff_angle=0, order=2, reshape=False):