How to add a footer to every new topic created (first post)?

I am looking for ways to add a small footer text to the first post in every topic created. I want it to show some general guideline information to people who may reply to the topic. So it should be at the end of the content the user posts in the topic she creates. Can someone help me with how to do it?

The best workaround for this would be to make a topic template (go into the Edit modal of a specific category) and explain this convention there.

Example:

<!-- Please write your post above this line and leave the content below intact -->

ALL REPLIES SHOULD BE IN ALL CAPS
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