I believe if you want to give it free, and allow others to change the code and give that for free, and you want to not be held liable under any sort of warranty, then you should just license the add-on under GNU General Public License v3 (GPL-3).
15. Disclaimer of Warranty. THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
16. Limitation of Liability. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.
Literally stating just that is pretty much good enough.
When you say...
'No support or warranty intended and no liability assumed by making this freely available, use at your own risk and have fun'
...there is no excuse for someone saying they didn't understand as such and saying that is much easier if all you want to do is give stuff away and that is understandable as you have said, you had it made so you probably can not support it yourself and that solidifies that they can take it as is or leave it and that one sentence intends that in the friendliest way you can say it.
There is no difference between a fancy purchased "NO TRESPASSING" sign and one you draw on a piece of cardboard and tape to a stick as long as the writing is clearly posted. If you own something, how you publicly declare it's acceptable use is on you. If it's free and you just want people to use it and mess around with it why not just keep it simple.
Especially since you can't really apply GPL to a XF add-on without the licence getting in the way for a variety of reasons (most notably the whole thing about 'if you bridge GPL code to non GPL code the resulting thing must be GPL')