docs: explain default of --min-py-version

[Cube707]

After our earlier discussion on #230 I now understood why the default is that way and why it makes sense to update require_python.

To avoid this problem in the future, I suggest to add a explanation to the README about the default behavior.

I hope this matches the conses reached on #207, where the mention of the specific default version was removed to reduce maintenance burdon

[asottile]

no thanks, you can check --help

[Cube707]

sorry for the maybe stupid question, but how does checking --help do that?

running setup-cfg-fmt --help on a v2.5.0 install results in:

usage: setup-cfg-fmt [-h] [--include-version-classifiers] [--min-py-version MIN_PY_VERSION] [--max-py-version MAX_PY_VERSION] [filenames ...]

positional arguments:
  filenames

options:
  -h, --help            show this help message and exit
  --include-version-classifiers
  --min-py-version MIN_PY_VERSION
  --max-py-version MAX_PY_VERSION