Specify behaviour of the precision parameter

[FabianInostroza]

The description of the precision parameter in the README states:

-p PRECISION, --precision PRECISION
smoothness for approximating curves with line segments
(float)

From that I would expect that higher precision values gives a better approximation of the curves but it is the opposite. Document the default value.

Also, a way to select a precision to start would be nice, at first I tried with precision = 1 and that took a long time to generate the footprint. I don't know how this could be solved.