[Docs] GroupsBy Example: Group by value of sum of columns
Hi,
TomAugspurger suggested me on [StackOverFlow](https://stackoverflow.com/questions/21584434/group-by-value-of-sum-of-columns-with-pandas) to suggest this example of `df.groupby()` here for being considered as relevant to add to the docs. The idea was to determine a way to regroup columns of a DataFrame according to their sum, and to sum the aggregated ones.

``` python
In [1]: import pandas as pd

In [2]: dat = {'a':[1,0,0], 'b':[0,1,0], 'c':[1,0,0], 'd':[2,3,4]}

In [3]: df = pd.DataFrame(dat)

In [4]: df
Out[4]: 
   a  b  c  d
0  1  0  1  2
1  0  1  0  3
2  0  0  0  4

In [5]: df.groupby(df.sum(), axis=1).sum()
Out[5]: 
   1  9
0  2  2
1  1  3
2  0  4
```
