ENH: Add .sem() to .describe()
Hi,
The standard error of the mean is quite an interesting statistic. But many didn't hear about it.
It would be nice to add it to ``.describe()`` hoping that people would ask themself about the difference between ``.sem()`` and ``.std()``

```python
series = pd.Series([1,2,3,4])
print(series.sem())
print(series.describe())
```

If many people like the idea then I would create a PR to add this feature :) 