einsum documentation addition

I really like einsum, very powerful, fast, flexible, and easy to read once you get the hang of it.  After reading the docs, I didn't know that you could leave subscripts empty for a scalar.  Specifically, np.einsum(',ij', 3, c) is equivalent to np.einsum('..., ...', 3, c), which I thought was pretty neat and not otherwise obvious in the docs.  Perhaps add that to the docs.  Thank you
