Update README.rst (#141)

README.rst

@@ -10,7 +10,7 @@
 This library provides run-time type checking for functions defined with
 `PEP 484 <https://www.python.org/dev/peps/pep-0484/>`_ argument (and return) type annotations.
 
-Three principal ways to do type checking are provided, each with its pros and cons:
+Four principal ways to do type checking are provided, each with its pros and cons:
 
 #. the ``check_argument_types()`` and ``check_return_type()`` functions: