Document how to compute encoder resolution

[calcmogul]

This is useful for figuring out what CPR encoder should be used for a mechanism, and that same information is needed for working autonomous software. An example would be:

distance per pulse = 1/(counts per rotation) * (2pi * diameter) meters per rotation * gear ratio from encoder to wheel

[modelmat]

I think also should show how to determine how accurate the velocity can be — which includes dividing by the control period.

My initial proposal for this was in terms of determining how accurate you could feasibly get control loops - If your rotations/s per pulse error is 100RPM it's not conceivable to get accuracy better than that.

(Though we are admittedly missing an article explaining how to calculate distance per pulse entirely too)