document how to write a new gumath kernel

[rgommers]

From @skrah I understood that writing a new kernel easily is a design goal and an important advantage over NumPy. It looks like this goal was achieved, however it would be good to validate that (with someone fairly new to gufuncs/XND/NumPy), and then document how to do this in the XND docs. @andrewfulton9's work on adding an MKL kernel (xnd-project/libgumath#37) will be a good validation.