Get a new project logo

[michaelosthege]

We should replace the Aesara logo which is still shown on the README with a new logo that does not infringe the Aesara brand.

[fonnesbeck]

I will start a contest for this. My inclination is to ask for something that plays off of the PyMC logo thematically somehow (rockets, colors, etc.) to emphasize the link between the two libraries. OTOH we could go for something entirely different if that's what people want. Thoughts?

[michaelosthege]

closed by #142