Documentation: Making a "flavor" python package

[Zsailer]

Reflecting back on this #3, it's not clear from the README how to make a pandas "flavor" that's importable.

The README shows how to register functions to DataFrames, but I think it needs documentation on how to write these functions in a module/package that you can import.

Something like:

1. Add "flavoring" functions in a module or package file my_flavor.py

   import pandas_flavor as pf
   
   @pf.register_series_method
   @pf.register_dataframe_method
   def cool_func(df, arg):
       print(arg)
       return df 

2. Import Pandas and that "flavor" module/package in a Python session.

   import pandas as pd
   import my_flavor
   
   df = pd.DataFrame(...).cool_func()