Updated readme with more details

Getting up and running wasn't the breeze I'd expected :smile:.

Couple of hours of playing and I'm now up and running and I thought I'd make my first contribution saving other people the hoops I jumped through.  I've updated the readme.md with the steps I followed to get up and running. I started with a clean install of Node on my machine and went from there.  

Hopefully people can have an easier "on ramp" with these amends?

By the way I dropped usage of "$" prefix in the node sample code bits and pieces as doesn't match all node environments; specifically Windows uses ">" which confused me for a while!  Hope that's okay.

Oh one more thing.  The installation that I went through changes I made added the following dependencies to ```packages.json```.  I haven't included this in the PR.  Perhaps I should?

```js
  "dependencies": {
    "docpad": "~6.64.0",
    "docpad-plugin-eco": "~2.0.3"
  },
```