Add commandline installation instructions to pugjs.org #3132

[TeraKelciPC]

This was posted long time ago at this link, reposting.
pugjs/pug#3132

Hey, I spent 1 hour to find out why my Webstorm File Watcher could not detect pug. I had installed Node.js, Express.js.
What I was missing was commandline installation instruction, which was not visible in the website.

Only in the github website I found it, and at the the I didn't even know what the question and solution was supposed to be.

So could you please also write the Command Line installation instructions from Github shown below in your website, that would save a lot of time. Thanks.
https://pugjs.org/api/getting-started.html

Command Line
After installing the latest version of Node.js, install with:

$ npm install pug-cli -g
and run with

$ pug --help