add python interop example (#33)

Author: ZuseZ4

src/SUMMARY.md

@@ -8,6 +8,7 @@
   - [Forward Mode](./usage/fwd.md)
   - [Reverse Mode](./usage/rev.md)
   - [Higher Order Derivatives](./usage/higher.md)
+  - [Python Integration](./usage/python.md)
 - [Current Limitations](./limitations.md)
   - [Future Work](./future_work.md)
 - [History and ecosystem](./ecosystem.md)

src/usage/python.md

@@ -0,0 +1,35 @@
+### Python Integration
+
+std::autodiff is fully handled by the Rust compiler and therefore should not cause any issues with Python integration.
+An example for maturin/PyO3 is provided below. You will still need to enable `lto=fat` in your Cargo.toml and adjust 
+the module name to match your project, otherwise python won't be able to find your functions.
+The first `#[pyfunction]` macro will only be applied to the original function `f`. 
+We therefore add a small wrapper function `df_py` and apply the `#[pyfunction]` macro to it.
+
+```toml
+
+```rs 
+#![feature(autodiff)]
+use std::autodiff::autodiff;
+use pyo3::prelude::*;
+
+#[pyfunction]
+#[autodiff(df, Reverse, Active, Active)]
+fn f(x: f32) -> f32 {
+    x * x
+}
+
+// Will return x*x and 2*x
+#[pyfunction]
+fn df_py(x: f32) -> (f32, f32) {
+    df(x, 1.0)
+}
+
+// Remember to adjust the name of the module to match your project
+#[pymodule]
+fn my_module(m: &Bound<'_, PyModule>) -> PyResult<()> {
+    m.add_function(wrap_pyfunction!(f_py, m)?)?;
+    m.add_function(wrap_pyfunction!(df_py, m)?)?;
+    Ok(())
+}
+```