More visibility about pixie backend - add note in the readme?

[pietroppeter]

I was looking on how to activate pixie backend and the only place I found is a note in the changelog about using backend parameter in ggsave. Probably we could add a note/section in the readme?

context: https://forum.nim-lang.org/t/9740#64103

[Vindaar]

One major reason why it's still so hidden is that it's still very much broken. The essentials are all there, but it doesn't work properly. Most of that is due to API changes in Pixie I imagine. There was a time when I tested it and it worked before the PR was merged. By the time of the merge it didn't work for me anymore. I started fixing it, but didn't fully finish it. I'll try to PR the changes I have so far one of these days to see where we're at.

[pietroppeter]

Ok, understood. I also want to try it and will let you know