signed-off commits

Our CONTRIBUTING.MD currently lists “signed-off” commits as a possible alternative to signing the Cesium CLA. Since we started allowing them, only one PR has been opened using them, and that PR didn’t even end up going into master.

In order to simplify and streamline the submission process, we are removing the option to use “signed-off” commits and instead encourage everyone who wishes to contribute to sign our CLA.

I’ll also add that for those that can’t or do not wish to sign the CLA, our new plug-ins repository is another alternative that further lowers the barrier to entry: