Add GPL rider that prohibits GitHub hosting

See https://thelig.ht/code/dbxfs/file/LICENSE.html via @pabs3

> dbxfs is generally licensed under the GPLv3+. Additionally it has the
> restriction that this code cannot be hosted on GitHub. In the instance
> where the prohibition of hosting this code on GitHub conflicts with
> the text of the GPLv3+, the prohibition of hosting this code on GitHub
> takes precedence.

Note that a license notice is not code, therefore I think it is OK to host the license notice here for detection purpose, but I am reaching out to the author to ask for permission