docs: Adding small environment variables section to readme (#1513)

README.md

@@ -338,6 +338,22 @@ The default templates comes with a `.css` file for each component. You can start
 - `npm install --save-dev less less-loader` (inside your preact application folder)
 - start replacing `.css` files with `.less` files
 
+### Using Environment Variables
+
+You can reference and use environment variables in your `preact.config.js` by using `process.env`:
+
+```js
+export default {
+	webpack(config, env, helpers, options) {
+		if (process.env.MY_VARIABLE) {
+			/** You can add a config here that will only used when your variable is truthy **/
+		}
+	},
+};
+```
+
+If you'd like to use these variables in your application, you can use the [DefinePlugin] config from our recipes wiki.
+
 [promise]: https://npm.im/promise-polyfill
 [fetch]: https://github.com/developit/unfetch
 [preact]: https://github.com/preactjs/preact
@@ -366,6 +382,7 @@ The default templates comes with a `.css` file for each component. You can start
 [`async!`]: https://github.com/preactjs/preact-cli/blob/1.4.1/examples/full/src/components/app.js#L7
 [sass]: https://sass-lang.com
 [less]: http://lesscss.org
+[defineplugin]: https://github.com/preactjs/preact-cli/wiki/Config-Recipes#use-environment-variables-in-your-application
 
 ### Route-Based Code Splitting