Ignite Getting Started confuses developers

By some reason, we suggest to build Ignite from sources at the very beginning of the getting started guide:

https://apacheignite.readme.io/docs/getting-started

I got a feedback that this confuses a lot especially because it's 100% optional! The reporter wasted much of his time trying to build Ignite with JDK 9 and could succeed only after a private conversation with me.

Revisit and rework the guide.