Not clear which CDN you are referring to. The link you provided is for
including in a WAR - as Play has already done for you in the public dir.
Now if you want them to be hosted on a read CDN (not GAE), then you can
copy out the files and change all references to point to the CDN. Perhaps
someone may have written a module for it but it is simple: just change the
URL that references each file in /public and point it to your CDN URL.