Pose Plugin needs work (or better instructions)

The pitch/roll/yaw output of the pose plugin doesn't make sense.

For example, a passport photo of a subject directly facing the camera returns:

`{pitch: 17.65802892174463, roll: -2.0460590975469586, and yaw: -24.25048987235748}`

If the pose orientation is based on something other than the camera's local vector space, there should be a notation in the documentation.