Update mypy_plugins.rst

Author: sobolevn

docs/pages/contrib/mypy_plugins.rst

@@ -15,15 +15,15 @@ You will need to install ``mypy`` separately.
 It is not bundled with ``returns``.
 
 To install any ``mypy`` plugin add it
-to the ``plugins`` section of the config file.
+to the ``plugins`` section of the config file (``setup.cfg`` or ``mypy.ini``):
 
 .. code:: ini
 
   [mypy]
   plugins =
     returns.contrib.mypy.returns_plugin
 
-Or
+Or in ``pyproject.toml``:
 
 .. code:: toml