DOC:Pandas groupby agg example for k-hot encoding
#### Location of the documentation
https://pandas.pydata.org/pandas-docs/version/0.22.0/generated/pandas.DataFrame.aggregate.html#pandas.DataFrame.aggregate

[this should provide the location of the documentation, e.g. "pandas.read_csv" or the URL of the documentation, e.g. "https://dev.pandas.io/docs/reference/api/pandas.read_csv.html"]

**Note**: You can check the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/).

#### Documentation problem
It is strange that even on stackoverflow reverse explode has only a single answer(that too is asked very recently). So adding a few more examples should not be bad.
![image](https://user-images.githubusercontent.com/40588378/97079156-51586480-160f-11eb-8545-2efa6c384784.png)

[this should provide a description of what documentation you believe needs to be fixed/improved]

#### Suggested fix for documentation
The following example tells how to perform k-hot encoding with agg. 

![image](https://user-images.githubusercontent.com/40588378/97079098-ddb65780-160e-11eb-9db3-0577a034352e.png)


```
maximum = df.groupby("ImageId").agg({"ClassId":lambda x:x.tolist()})["ClassId"].max()[0]
df = df.groupby("ImageId").agg({"ClassId":lambda x:x.tolist()})
df["ClassId"] = df["ClassId"].apply(lambda x:[1 if i+1 in x else 0 for i in range(maximum)] )
```

![image](https://user-images.githubusercontent.com/40588378/97079113-09394200-160f-11eb-90ad-5b1c38d86859.png)

[this should explain the suggested fix and **why** it's better than the existing documentation]
