Document configuration required to allow file exclusion via config file

[jimrybarski]

Description

This adds a line to the README to clear up a common point of confusion.

Motivation and Context

pre-commit passes all source files as arguments to interrogate. Interrogate ignores any exclusion criteria in pyproject.toml or setup.cfg for files that are specified in this way. This gives the appearance that interrogate is ignoring configuration options. This has led to a number of issues, in particular #60, #100, #145, #155, and possibly #128.

Have you tested this? If so, how?

I have used this exact line in my own project and it works as intended.

Checklist for PR author(s)

• [x] Changes are covered by unit tests (no major decrease in code coverage %).
• [x] All tests pass.
• [x] Docstring coverage is 100% via tox -e docs or interrogate -c pyproject.toml (I mean, we should set a good example :smile:).
• [x] Updates to documentation:
  • [x] Document any relevant additions/changes in README.rst.
  • [x] Manually update both the README.rst and docs/index.rst for any new/changed CLI flags.
  • [x] Any changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives. Find the appropriate next version in the project's __init__.py file.

Release note

NONE