docs: Add note on additional scope for private channels

[mikebryant]

What does this PR do?

Adds a missing scope to the README

Motivation

Tried to use it as described, failed with:

slack.errors.SlackApiError: The request to the Slack API failed.
The server responded with: {'ok': False, 'error': 'missing_scope', 'needed': 'groups:history', 'provided': 'channels:history,reactions:read,reactions:write'}

Additional Notes

Possible Drawbacks / Trade-offs

Describe how to test/QA your changes