df.explode is required for reset_index
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
A = pd.DataFrame({'x': [['hoge', 'piyo'], ['fuga']], 'y': ['1', '2']})
B = pd.DataFrame({'x': [['hoge2', 'piyo2'], ['fuga2'], ['meta2']], 'y': ['1', '2', '3']})
A = A.append(B)
```
```
# >>> print(A)

    x  y
0    [hoge, piyo]  1
1          [fuga]  2
0  [hoge2, piyo2]  1
1         [fuga2]  2
2         [meta2]  3
```
```
# >>> A.explode('x')
        x  y
0   hoge  1
0   piyo  1
0  hoge2  1
0  piyo2  1
0   hoge  1
0   piyo  1
0  hoge2  1
0  piyo2  1
1   fuga  2
1  fuga2  2
1   fuga  2
1  fuga2  2
2  meta2  3
```

#### Problem description

The ideal of `A.explode('x')` output is following.
```
      x  y
hoge  1
piyo  1
fuga  2
hoge2 1
piyo2 1
fuga2 2
meta2 3
```

But, The actual output is enlarged because the index of `DataFrame A`.  
This will cause mistakes for users of explode method.

#### Expected Output

The solution is to do a `reset_index` before the `explode`.
```
# >>> A.reset_index(drop=True).explode('x')
         x  y
0   hoge  1
0   piyo  1
1   fuga  2
2  hoge2  1
2  piyo2  1
3  fuga2  2
4  meta2  3
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.8.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 18.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : ja_JP.UTF-8
LOCALE           : ja_JP.UTF-8

pandas           : 0.25.3
numpy            : 1.16.4
pytz             : 2018.9
dateutil         : 2.7.5
pip              : 19.2.3
setuptools       : 41.0.1
Cython           : 0.29.4
pytest           : 4.4.0
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.3.3
html5lib         : None
pymysql          : None
psycopg2         : 2.8.1 (dt dec pq3 ext lo64)
jinja2           : 2.10.1
IPython          : 7.4.0
pandas_datareader: None
bs4              : 4.7.1
bottleneck       : None
fastparquet      : None
gcsfs            : 0.3.0+7.g59da8cd
lxml.etree       : 4.3.3
matplotlib       : 3.0.3
numexpr          : None
odfpy            : None
openpyxl         : 2.6.0
pandas_gbq       : 0.10.0
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.3.0
sqlalchemy       : 1.3.4
tables           : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None

</details>

Sorry for ugly English. Thanks.
