Add documentation for tz.gettz

Add changelog for PR #704
diff --git a/docs/tz.rst b/docs/tz.rst
index 5fd5bf5..0dca230 100644
--- a/docs/tz.rst
+++ b/docs/tz.rst
@@ -4,3 +4,5 @@
 .. automodule:: dateutil.tz
    :members:
    :undoc-members:
+
+.. automethod:: dateutil.tz.gettz
\ No newline at end of file