DOC: add cookbook example

cpcloud · cpcloud · commit b2ad0442e932 · 2013-08-12T20:00:38.000-04:00
diff --git a/doc/source/cookbook.rst b/doc/source/cookbook.rst
@@ -126,6 +126,9 @@ Fill forward a reversed timeseries
    df
    df.reindex(df.index[::-1]).ffill()
 
+`cumsum reset at NaN values
+<http://stackoverflow.com/questions/18196811/cumsum-reset-at-nan>`__
+
 Replace
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