More docs for "Python in Rust"

[test3211234]

I'm not even sure whether to use a venv or not and how, it doesn't say.

[shenjackyuanjie]

yes, currently "python in rust" is still lack of docs and configuring python runtime env is not a easy thing ( require some manual pyo3::ffi function call ) about using venv, you can checkout this code for a refrence