a better notation in the README

[thudjx]

As writes in the README, when the integrand is multi-dimensional, it should be passessed as f(x, v), and it should be noted that v is assigned in f. I am thinking it is better to name the integrand with an additional ! suffix to remind the fact that f changes its arguments. And the notation in integrals of vector valued integrands of the README would be more straitforward to julia users.