Apple requires contributors to iOS projects to relicense their code on submit. We'll have to have individual contributors sign something to enable this.
Our volunteer legal have put together a form you can sign electronically. So no scanning, faxing, or carrier pigeons involved. How modern: https://whispersystems.org/cla/
Please go ahead and sign, putting your github username in "Address line #2", so that we can accept your pull requests at our heart's delight.
Follow all Signal-iOS Code Conventions.