Update README.md

[mcabbott]

This alters the readme to:

• Show all 4 basic functions
• Not use random numbers, but instead something interpretable
• Link to DiffRules, and to JuliaDiff's round-up of other AD packages
• Not define a g = x -> ForwardDiff.gradient(f, x), as it seems like poor style to define a function like this?
• Remove note about upgrading, since earlier versions were a long time ago. (Still covered in docs. Maybe g = x -> ... was to illustrate a change.)