Clarify custom license

LICENSE_CUSTOM

 This project is generally licensed under the GPLv3. However, if this license doesn't match your requirements, or prevents you from using
 this code otherwise, contact the developer at daniel.wolf@frostnerd.com. A license exception and/or purpose-bound modification to the license might
-be granted then for parts of the app.
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+be granted then for parts of the app (only parts which no other developers have been working on can be granted an exception).
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