text: Title: [DOC] API Policy
  Body: I would like to put the following API policy in place in the docs:
  
  > pyjanitor only extends or aliases the pandas API (and other dataframe APIs), but will never fix or replace them.
  > Undesirable pandas behaviour should be reported upstream in the pandas repository.
  > If at some point the pandas devs decide to take something from pyjanitor and internalize it as part of the official pandas API, then we will deprecate it from pyjanitor, while acknowledge the original contributors' contribution as part of the official deprecation record.
  
  Taken from [here](https://github.com/ericmjl/pyjanitor/issues/612#issuecomment-557250963) in issue #612.