update environment variables docs to explain how to implement dotenv

docs/docs/environment-variables.md

@@ -20,9 +20,16 @@ normal ways e.g. when calling gatsby on the command line or by adding
 environment variables through your hosting/build tool.
 
 If you want to access variables in `.env.*` files in your node.js code, use the
-NPM package [dotenv](https://www.npmjs.com/package/dotenv). Once you've
-installed dotenv and followed their setup instructions, you can use your
-environment variables in the same way as shown in the example below.
+NPM package [dotenv](https://www.npmjs.com/package/dotenv). Install the package and 
+require it in your `gatsby-config.js` or `gatsby-node.js` the following way on top of your file:
+
+```
+require('dotenv').config({
+  path: `.env.${process.env.NODE_ENV}`
+});
+```
+
+Now the variables are available.
 
 ## Example