I want to concatenate two dataframes (df1, df2) in Pandas (python)
such that the result would contain all the indices (unique and common) of df1 and whatever df2 has but isn't there in df1. An example:

**df1**

```

     col1 col2
0     1   2
1     2   3
2     3   4

```

**df2**

```

     col1 col2
1     4   6
2     2   3
3     5   5

```

I want the result to be like:

```

     col1 col2
0     1   2
1     2   3
2     3   4
3     5   5

```