Update README

[okonet]

I'm wondering if all steps listed in the README are still required to get it running. @derekrushforth could you review it please when you get time?

[tomholland]

Might be worth documenting what version of Node.js is required too, for the forkers; I had v4.4.5 ("Recommended For Most Users"); had to update to v6.2.0 in order to stop getting syntax errors.

[okonet]

We have .nvmrc for that purpose but it's definitively not bad to mention it