Add JavaScript config example to README

Author: aarongoldenthal

README.md

@@ -18,8 +18,9 @@ Pa11y CI runs accessibility tests against multiple URLs and reports on any issue
 * [Requirements](#requirements)
 * [Usage](#usage)
   * [Configuration](#configuration)
-  * [Default configuration](#default-configuration)
-  * [URL configuration](#url-configuration)
+    * [Default configuration](#default-configuration)
+    * [URL configuration](#url-configuration)
+    * [Javascript configuration](#javascript-configuration)
   * [Sitemaps](#sitemaps)
 * [Reporters](#reporters)
   * [Use Multiple reporters](#use-multiple-reporters)
@@ -78,7 +79,7 @@ You can use the `--config` command line argument to specify a different file, wh
 
 Pa11y will be run against each of the URLs in the `urls` array and the paths specified as CLI arguments. Paths can be specified as relative, absolute and as [glob](https://github.com/isaacs/node-glob#glob) patterns.
 
-### Default configuration
+#### Default configuration
 
 You can specify a default set of [pa11y configurations] that should be used for each test run. These should be added to a `defaults` object in your config. For example:
 
@@ -103,7 +104,7 @@ Pa11y CI has a few of its own configurations which you can set as well:
 * `concurrency`: The number of tests that should be run in parallel. Defaults to `1`.
 * `useIncognitoBrowserContext`: Run test with an isolated incognito browser context, stops cookies being shared and modified between tests. Defaults to `true`.
 
-### URL configuration
+#### URL configuration
 
 Each URL in your config file can be an object and specify [pa11y configurations] which override the defaults too. You do this by using an object instead of a string, and providing the URL as a `url` property on that object. This can be useful if, for example, you know that a certain URL takes a while to load or you want to check what the page looked like when the tests were run:
 
@@ -123,6 +124,26 @@ Each URL in your config file can be an object and specify [pa11y configurations]
 }
 ```
 
+#### Javascript configuration
+
+If a JavaScript configuration file is used, it should be a CommonJS module that exports a configuration object. This can be used to dynamically update configuration parameters, for example taking data from environment variables as shown in the example below.
+
+```js
+module.exports = {
+    defaults: {
+        timeout: 1000,
+        viewport: {
+            width: 320,
+            height: 480
+        },
+        headers: {
+            token: process.env.TOKEN
+        }
+    },
+    urls: ["https://pa11y.org/", "https://pa11y.org/contributing"]
+};
+```
+
 ### Sitemaps
 
 If you don't wish to specify your URLs in a config file, you can use an XML sitemap that's published somewhere online. This is done with the `--sitemap` option: