Add Sage feature, with a badge

[rbeezer]

@davidfarmer If you sent me a proper graphics file for Sage (off-list?), I could do the rest to add this in generally, and add to a few choice projects. Not sure why I did not think of this sooner...

[kcrisman]

Definite 👍 for this from me, given that I will be advertising to Sage fans tomorrow ... not that it would be implemented that fast 😉

[kcrisman]

See here, perhaps? https://www.sagemath.org/library-marketing.html