Add FAQ entry about scipy.special

[seibert]

Thanks to this project:

https://numba-special.readthedocs.io/en/latest/index.html

There are now wrappers for the scipy.special functions. We should mention this project in our FAQ.

As a side question: This implementation requires individually extracting all the function pointers using a .pyx file:

https://github.com/person142/numba_special/blob/master/numba_special/function_pointers.pyx

I wonder if there is a way to do that more automatically...

[stuartarchibald]

See "scumba" discussion here: #4292

[stuartarchibald]

The Numba extension project now supports this feature: https://github.com/numba/numba-scipy this should be mentioned instead.