Fix README.md once more.
Fix README.md once more.

Removed method keywords, clarified the reasons for use of user provided gradients, and clarified a dispatch comment.
pkofod/Optim.jl,JuliaPackageMirrors/Optim.jl,matthieugomez/Optim.jl,matthieugomez/Optim.jl,pkofod/Optim.jl,JuliaPackageMirrors/Optim.jl