Update README.rst

Author: bckohan

README.rst

@@ -106,6 +106,8 @@ accessible as their enumeration type rather than by-value:**
 `enum-properties <https://pypi.org/project/enum-properties/>`_ which makes
 possible very rich enumeration fields.
 
+`pip install enum-properties`
+
 .. code:: python
 
     from enum_properties import s