Before pull requests can be accepted, the contributor needs to sign a contributor license agreement. (Essentially giving a license for using his code and/or media and for re-licensing it, for details read the agreement linked below.)
To keep the hassle for everyone involved to a minimum, FlatBedrock uses CLAHub to manage the agreements. To get started, sign the Contributor License Agreement.
Without CLAs it at some point becomes essentially impossible to change the license of a project which accepts contributions from third parties. Without an explicit agreement every contributor would still own the copyright to his code. So as long as it (or parts of it) remain in the code base, he would have to agree to every license change. If enough time has passed, there may be dozens, perhaps even hundreds of contributors involved and it will be very hard to track down each and every one of them and get them to agree on a new license.
This would make it impossible to change the license to a more permissive one at a later date, so the code would be locked into CC BY-NC-ND 3.0 until it passes into the public domain about a hundred years from now. This situation is to be avoided and that’s why you are being bothered with this CLA.