Add CIF module to be installed in the environment in README file

[mairagatti]

Traceback (most recent call last):
File "./main.py", line 22, in
from cgcnn.data import CIFData
File "cgcnn/data.py", line 19, in
from CifFile import ReadCif
ModuleNotFoundError: No module named 'CifFile'

https://github.com/txie-93/cgcnn/blob/master/README.md

[sayantan1410]

Hello, I would like to contribute to this issue, can you explain a bit more what I have to do.

[txie-93]

@mgdebayser @sayantan1410 Do you both get this error? I don't see this import from CifFile import ReadCif in line 19 of data.py