There is a pandas data frame:

```
df = pd.DataFrame({'c1':['a','b','c','d','e','f','g','h','i','j'],
                   'c2':[10,12,23,4,18,98,11,23,33,99]})

    c1  c2
0   a   10
1   b   12
2   c   23
3   d   4
4   e   18
5   f   98
6   g   11
7   h   23
8   i   33
9   j   99

```

I want to create a new data frame that only contains the top 20% rows according to values in column c2, in this case:

output:

```
   c1   c2
0   f   98
1   j   99

```