Document how to exclude exceptions coming from using assert (#224)

Updates the README.md file to mention the special use case when
attempting to ignore `AssertionErrors` coming from the use of the
`assert` keyword.

Signed-off-by: Rémy Greinhofer <remy.greinhofer@gmail.com>

Author: rgreinho

README.rst

@@ -107,6 +107,13 @@ would only rerun those errors that does not match with ``AssertionError`` or ``O
 
    $ pytest --reruns 5 --rerun-except AssertionError --rerun-except OSError
 
+.. note::
+
+   When the ```AssertionError``` comes from the use of the ``assert`` keyword,
+   use ``--rerun-except assert`` instead::
+
+   $ pytest --reruns 5 --rerun-except assert
+
 Re-run individual failures
 --------------------------