"Add a tag demo" rule

[jbmoelker]

Summary

Proposed changes:

• "Add a tag demo" rule (incl. summary, why, how and example)

  Checklist

• [X] The changes only affect or contain one style guide rule or section.
• [X] The changes are in style guide format.
• [X] The new style guide section has an entry in the Table of Contents (only if changes introduce new section).
• [X] The changes can be merged into the target branch without conflicts.

[jbmoelker]

tested: http://plnkr.co/edit/aEIQj7?p=preview