That's fine, and it is of course how many projects use the GPL in most cases in practice -- but as it reads in the readme, it sounds like the GPL doesn't [allow] someone to fork the project, port it to say, OS X, or arm - and sell the changed fork to a to a customer without giving the changes back upstream. The porter would have to offer sources to the customer, and the customer would be free to upstream the sources - but from the GPL, there's no legal compulsion to do so.

Anyway, I guess I would have reworded it somewhat, to make it more obvious that the source is under GPL, but that the project welcomes and encourages upstreaming changes. This opposed to the code being under a modified GPL.