SettingWithCopyWarning when modifying a column added to df after read_csv
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
df = pd.read_csv('package_infos.csv')
m = df.shape[0]
df['etags'] = pd.Series(dtype=str, index=range(m))
df['etags'][0] = "Hello world"
```

Download package_infos.csv from here: https://raw.githubusercontent.com/jamesqo/nuget_recommender/b05ef43a01e54b61851f51708cdd6dce3b2afe33/package_infos.csv

Doing `df.loc[0, 'etags'] = "Hello world"` seems to remove the warning, but it's slowing down my app significantly.