Add note about NVM and change one paragraph header
Add note about NVM and change one paragraph header

Ref. https://github.com/arkency/reactjs_koans/issues/45
kevmo/reactjs_koans,pandemiccreative/reactjs_koans,nlhuykhang/reactjs_koans,erossini/reactjs_koans,gregRV/react_koans,arkency/reactjs_koans,hnq90/reactjs_koans,nlhuykhang/reactjs_koans,kevmo/reactjs_koans,erossini/reactjs_koans,pandemiccreative/reactjs_koans,hnq90/reactjs_koans,gregRV/react_koans,arkency/reactjs_koans