Change license or setup CLA for merge requests

I didn't want to add a CLA, really did not, but I just learned that Apple's App Store's Terms of Use are not compatible with GPL 3.0, and eventually I want to publish Manyverse for iOS, which only allows the Apple App Store for distribution to end users. The way of having a GPL software on the App Store appears to be (as far as I know this is the only way, I'm glad to be shown otherwise) have a Contributor's License Agreement so that every contributor agrees to basically "not take the project hostage" using legal action. Upon request, Apple seems to easily remove a GPL app from App Store.

I still want to figure out what is the right CLA for this, but so far here are some references that may help me on the way there:

• https://github.com/peachcloud/meta/blob/master/processes/cla.md
• http://www.harmonyagreements.org/index.html