This section is actually redundant with the "building with maven" section about Hadoop versions. Maybe it would make sense to just have a short introduction here that explains that (i) you need to have a version of spark that is specially compiled with YARN support if you don't already and (ii) if you don't have one go to the maven build to learn how to make one.

I think right now if users go to this, the first thing they'll think is they have to go build Spark. But actually in almost all cases they can just download the pre-build yarn jar and be done with it. I think the first draft of this document was when we didn't package a binary with YARN.