Specify version requirements in contributing.md

[saadq]

Because there's a fair amount of new contributors, and I've had some users in the past, there should probably be a notice in the contributing.md which mentions that Node v7.6+ is required and npm v5+ is required.

[esslamben]

I can look into this :)

[alexluong]

Sorry if this is not the right place to talk about it, but is there any good issue you may need help with, @saadq? I'm comfortable with React and Node, so I should be able to tackle or inspect anything.

[saadq]

@alexluong I can definitely use help from someone who has React/Node experience! There are some things that probably aren't listed in the issues so I'll ping you once I have those issues created and you can let me know if any of them interest you.

[alexluong]

@saadq Sounds great. Just let me know in the issues when you create them and I'll take a look!

[saadq]

Fixed by #79