Emphasise need for node-gyp for GNU/Linux and MacOS in node.js installation

...and add in instructions for relevant node install:

curl https://deb.nodesource.com/gpgkey/nodesource.gpg.key | sudo apt-key add -

sudo apt-add-repository "deb https://deb.nodesource.com/node_7.x $(lsb_release -sc) main"

sudo apt-get update

sudo apt-get install nodejs