The following is the suggested text for putting a ruleset under the GNU Free Documentation License. Typically you would place the following copyright notice at the front of your ruleset and the full text of the licence in its own section at the end. Find out more about the GNU FDL.

Copyright © <YEAR> by <DESIGNER-NAME>. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".