Updated README to clarify default action, not functions as an override, and to clarify default delimiter.

[dmikusa]

Summary

The behavior of default (i.e. no action set) has changed.

Use Cases

The default behavior when an action is not set should match up with the spec, which says when there is no suffix present, the behavior is to override.

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• [n/a] I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.