Add docs for date_modify

diff --git a/doc/filters/date_modify.rst b/doc/filters/date_modify.rst
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+``date_modify``
+===============
+
+.. versionadded:: 1.9
+    The date_modify filter has been added in Twig 1.9.
+
+The ``date_modify`` filter modifies a date with a given modifier string:
+
+.. code-block:: jinja
+
+    {{ post.published_at|date_modify("+1 day")|date("m/d/Y") }}
+
+The ``date_modify`` filter accepts strings (it must be in a format supported by the
+`strtotime`_ function) or `DateTime`_ instances. You can easily combine it with the `date`_
+filter for formatting.
+
+.. _`strtotime`:    http://www.php.net/strtotime
+.. _`DateTime`:     http://www.php.net/DateTime