Document lightning requirement for npm

[grasmash]

We must note that if you are using lightning, the system requirements include NPM.

[aweingarten]

@grasmash, @sarahjean
Does it make sense to maybe add a task or a target-hook to install node? This is a recurring headache every-time we spin up a new codebase. Would be nice if it just worked.

[grasmash]

@aweingarten I've explored that, but found that thar be dragons. Much better to have this handled on a per machine basis. Feel free to submit a PR if you'd like to tackle it.