New icon

[GijsTimmers]

Let me first say that I really appreciate all the efforts that are going into Sonata. It's a great mpd client and I will not start using another client, no matter the outcome of this issue.

With that said, I think Sonata's icon can be improved. Currently, it's a music note, which is a little bit boring, to be honest, and also, there are lots of other music players that use the same music note more or less.

Now, I'm a guy who likes aesthetics but can't make icons himself. My proposition is to attract graphic designers by mentioning this on the README.md.