ENH: Added 'sum' to the set of statistics returned by df.field.describe()
I've added ``sum`` to the set of statistics returned by the ``describe`` method.

I believe it should be added because in my experience, the sum is just as commonly needed as the other values already returned by ``describe``. I have often found myself writing an additional line of code after ``describe`` to calculate the sum of the same series. Adding sum to the mix would remove that requirement and make life easier for myself and others.

I believe I am not alone. For instance, ``sum`` is already included in the descriptive statistics methods of other programs, like Microsoft Excel.

 - [x] tests added / passed
 - [x] passes ``git diff upstream/master | flake8 --diff``
 - [ ] whatsnew entry

@palewire See discussion in https://github.com/pandas-dev/pandas/issues/14483 Thanks for the link. 

The discussion I see there seems like an argument for completely deprecating describe. 

I can respect that, but if you are going to keep it in the project, why not expand it to include common aggregations like sum, which is prominently featured in the examples for the agg expansion? Having both seems like a road to confusion for users. There are no plans to deprecate `describe`. And I also do not see the potential confusion. The addition of `agg` should be looked at like this (apart from other reasons to have `agg`):

- `describe` gives you a default set of statistics
- if you want a custom set of statistics, the `agg` method let's you rather easily create a function to do this

But of course, there can still be discussion about what the default set of statistics should be, and whether this should include a sum or not. But, what I just want to say is that, for me, this discussion is separate from the introduction of `agg`. 

There are some reasons listed in #14483 to not include sum in `describe`. If you have additional arguments to include it (eg the other programs you mention, although R is an example that does not include sum in its summary), please add it to that issues (to keep the discussion in a central place) @palewire 

closing this, but if you would like to raise an issue for discussion (xref the previous issues). with a convincing argument that would be fine.