Add usage instructions to the README

[bhrutledge]

I put a stub in #1. It could be something as simple as:

Run script_name.py -h to see usage.

However, it looks like there should be at least be something about the config file, because it errors right away without it:

% python generatetrelloreport.py -h
Traceback (most recent call last):
  File "/Users/bhrutledge/Dev/remove-trello-data/generatetrelloreport.py", line 6, in <module>
    from trelloeditutils import *
  File "/Users/bhrutledge/Dev/remove-trello-data/trelloeditutils.py", line 16, in <module>
    REQUESTS_ID = config['MAMAS']['REQUESTS']
  File "/opt/homebrew/Cellar/python@3.9/3.9.5/Frameworks/Python.framework/Versions/3.9/lib/python3.9/configparser.py", line 960, in __getitem__
    raise KeyError(key)
KeyError: 'MAMAS'

You could commit a config.ini.example, with placeholder values, and instructions to copy it to config.ini and add the real values (along with instructions on where to get those).