Doc outdated

I built and wrote some simple cases to try `std::autodiff` feature in rust. It seems that the doc is outdated. Some main issues are listed in the following:
- autodiff has been changed to `autodiff_reverse` and `autodiff_forward`.
- We do not have forward mode example in this doc.

Do you have plans to update it? If not, I can help update this doc and add some examples for forward mode. I will play with more features.