Added a parser example for overriding parserinfo

In conjunction with GH issue #426.
diff --git a/docs/examples.rst b/docs/examples.rst
index 2b98dbd..21270ca 100644
--- a/docs/examples.rst
+++ b/docs/examples.rst
@@ -1190,6 +1190,19 @@
     datetime.datetime(1990, 6, 13, 5, 50)
 
 
+Override parserinfo with a custom parserinfo
+
+.. doctest:: tz
+
+   >>> from dateutil.parser import parse, parserinfo
+   >>> class CustomParserInfo(parserinfo):
+   ...     # e.g. edit a property of parserinfo to allow a custom 12 hour format
+   ...     AMPM = [("am", "a", "xm"), ("pm", "p")]
+   >>> parse('2018-06-08 12:06:58 XM', parserinfo=CustomParserInfo())
+   datetime.datetime(2018, 6, 8, 0, 6, 58)
+
+
+
 tzutc examples
 --------------