Directly include instructions for Node v9 installation in readme, fix #247

Why this change?

To make development setup easier. :) I’m not such an experienced coder and copy-pasting commands is quite nice.

The original link https://nodesource.com/blog/installing-node-js-tutorial-using-nvm-on-mac-os-x-and-ubuntu/ shows people how to install Node 11 (latest) or 10 (long-term support), both of which are not supported by drip. So including the 2 commands which will set it up correctly seem much simpler.

(Also ref to my confusion in issue #247 (closed).)

Reviewing

Here's what I am looking for feedback on in particular:

• Is including these commands directly ok?
• Is 9 the preferred Node version?
• Does this work fine on macOS? I’m only on Ubuntu.