add note about serverside rendering in .tag.html rule

[petergoes]

Summary

Proposed changes:

• Add note aboute extensions in .tag.html rule as discussed in .tag.html extension for server-side rendered tags #57

Resolves #57 .

Checklist

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