guide: show exporting create_exception! exceptions in a module

guide/src/exception.md

@@ -33,6 +33,23 @@ fn main() {
 }
 ```
 
+When using PyO3 to create an extension module, you can add the new exception to
+the module like this, so that it is importable from Python:
+
+```rust,ignore
+
+create_exception!(mymodule, CustomError, PyException);
+
+#[pymodule]
+fn mymodule(py: Python, m: &PyModule) -> PyResult<()> {
+    // ... other elements added to module ...
+    m.add("CustomError", py.get_type::<CustomError>())?;
+
+    Ok(())
+}
+
+```
+
 ## Raising an exception
 
 To raise an exception, first you need to obtain an exception type and construct a new [`PyErr`], then call the [`PyErr::restore`](https://docs.rs/pyo3/latest/pyo3/struct.PyErr.html#method.restore) method to write the exception back to the Python interpreter's global state.