python3Packages: document how to handle extras-require

doc/languages-frameworks/python.section.md

@@ -979,6 +979,31 @@ with import <nixpkgs> {};
 in python.withPackages(ps: [ps.blaze])).env
 ```
 
+#### Optional extra dependencies
+
+Some packages define optional dependencies for additional features. With
+`setuptools` this is called `extras_require` and `flit` calls it `extras-require`. A
+method for supporting this is by declaring the extras of a package in its
+`passthru`, e.g. in case of the package `dask`
+
+```nix
+passthru.extras-require = {
+  complete = [ distributed ];
+};
+```
+
+and letting the package requiring the extra add the list to its dependencies
+
+```nix
+propagatedBuildInputs = [
+  ...
+] ++ dask.extras-require.complete;
+```
+
+Note this method is preferred over adding parameters to builders, as that can
+result in packages depending on different variants and thereby causing
+collisions.
+
 #### `buildPythonApplication` function {#buildpythonapplication-function}
 
 The `buildPythonApplication` function is practically the same as