ForwardDiff over closed interval

[ChrisRackauckas]

@weymouth

I see this change marked at the top of the README, but I don’t see any suggestions for how to get the derivative on the left of u or any other “designed” approach to deal with closed domains like this.

function LeftDeriv(f, x)
    ForwardDiff.derivative(t -> -f(-t), -x)
end