Fixed pandas-dev#12561: providing links to the .cum* in .expanding

 pandas-dev#12651 
commit for the .cum* in .expanding

Author: prabhjotsumman

doc/source/computation.rst

@@ -578,6 +578,15 @@ they are implemented in pandas such that the following two calls are equivalent:
    df.expanding(min_periods=1).mean()[:5]
 
 These have a similar set of methods to ``.rolling`` methods.
+Also ``.expanding().sum()`` has a similar functionality to ``.cumsum().ffill()`` 
+
+..ipython:: python
+	df.expanding().sum()
+	
+	df.cumsum().ffill()
+	df.cumprod().ffill()
+	df.cummax().ffill()
+	df.cummin().ffill()
 
 Method Summary
 ~~~~~~~~~~~~~~