Add links in main readme to autodiff resources for those who would like to read further

[kostaleonard]

As a machine learning engineer, I want to have links to reading resources on difficult topics like automatic differentiation so that I can learn more and do my own research. Some of these resources include:

• The autograd Python package.
• The autodidact Python package, which is a condensed version of autograd meant for teaching and learning automatic differentiation.
• Professor Roger Grosse's automatic differentiation lecture slides.
• The JAX autodiff cookbook.