Enable calc_misssing_rate work with pd.DataFrame

[WenjieDu]

What does this PR do?

1. fixing Make calc_misssing_rate work on pd.DataFrame #37;
2. update docs;

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.