I just added two alternatives, clementine and mopidy. the latter one I did not try miself, but it is packaged in the official repos (community) and it is advertised prominently on the product's homepage. Since there is no mopidy wiki article, I am not sure how much this info helps.. but maybe there is one in the future, then it could entail a section about spotify support.
There is a dependency on Zenity for the official client, it's needed when we want to add Local Files through the "ADD A SOURCE" button.
I think it should be informed somehow in the main page.