Document error bounds

[dmadisetti]

So Beatson/Greengard list the L1 error as bounded here:

$$L1 < frac{\sum_i |q_i|}{r-a} (\frac{a}{r})^{lmax+1}$$

But this is the exterior expansion. I don't have as many resources on the interior expansion.

I think it would be nice to show

• This project keeps these bound
• Add documentation on the bounds