Add example of #info

[jecisc]

I added DataFrame>>#info method printing some info such as:

DataFrame: 20640 entries
Data columns (total 10 columns):
 # | Column | Non-nil count | Dtype
---------------------------------------------------
1 | longitude | 20640 non-nil | SmallFloat64
2 | latitude | 20640 non-nil | SmallFloat64
3 | housing_median_age | 20640 non-nil | SmallFloat64
4 | total_rooms | 20640 non-nil | SmallFloat64
5 | total_bedrooms | 20433 non-nil | SmallFloat64
6 | population | 20640 non-nil | SmallFloat64
7 | households | 20640 non-nil | SmallFloat64
8 | median_income | 20640 non-nil | SmallFloat64
9 | median_house_value | 20640 non-nil | SmallFloat64
10 | ocean_proximity | 20640 non-nil | Object

Maybe this could have a spot in the documentation to help the user understand the structure of his dataframe