CREDITS: Convert into string resource

Created by: SecUpwN

Taken from https://github.com/SecUpwN/Android-IMSI-Catcher-Detector/pull/709#issuecomment-167266552, our CREDITS file is currently an asset instead of a string resource. It also contains manual line-breaks with \n. Since we've changed how our CREDITS are being displayed, we should make sure that this file is following the best practice for developers for easier editing and view.

@larsgrefer, would you please have a look at this Issue? Should we center all text as well?