Add a Slack Guideline

[tanto259]

Proposed changes

This PR adds a Slack guideline to the COBOL Programming Course's Code of Conduct as per the TAC discussion for reducing Slack channel pollution.

Type of change

What type of changes does your PR introduce to the COBOL Programming Course? Put an x in the boxes that apply.

• [ ] Bug fix (change which fixes one or more issues)
• [ ] New feature (change which adds functionality or features to the course)
• [ ] Translations (change which adds or modifies translations of the course)
• [x] Documentation (change which modifies documentation related to the course)
• [ ] This change requires an update to the course's z/OS environment

Checklist:

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This checklist will be used as reference for both the contributor and the reviewer

• [x] I have read the Contributing Guideline
• [x] I have included a title and description for this PR
• [x] I have DCO-signed all of my commits that are included in this PR
• [x] I have tested it manually and there are no regressions found
• [ ] I have commented my code, particularly in hard-to-understand areas (if appropriate)
• [ ] I have made corresponding changes to the documentation (if appropriate)