The user settings Signature Settings node already allows a node template to be provided. It wouldn't be hard to default that to some suitable preliminary text along the lines of "Please read Writeup Formatting Tips then delete this line.".

Icing would be to have PM notice that the template text remains at the start or end of the node and disallow create until it is removed. In that case we should either indicate in Writeup Formatting Tips or in the bullet list how to change the template text.