Clarify relation to NixOps

[copumpkin]

It seems like a pretty FAQ-ish thing, and I'd put it in the readme or the docbook.

[bsima]

+1 what exactly is the relationship with NixOps?

[svanderburg]

Maybe I should explicitly mention this in the manual. For the impatient, there is already a blog post elaborating on this: http://sandervanderburg.blogspot.com/2015/03/on-nixops-disnix-service-deployment-and.html