Add social media image

[morisil]

I just shared Polish translation on facebook and noticed that the default preview is not very spectacular because it's taken from the creativeCodeBerlin profile.

There is an option now to use "social media image" for GitHub projects. Any ideas for something representative for the whole topic, maybe a kind of miniatures? Here is how it looks when sharing my project:

https://github.com/xemantic/shader-web-background