Document Asynchronous Configurations in the Config File's Docs

docs/config/01-configuration-file.md

@@ -81,6 +81,36 @@ require('ts-node').register({
 require('./karma.conf.ts');
 ```
 
+### Asynchronous Configuration
+
+The function exported by the configuration file may return a promise whose
+resolution will tell Karma when all configuration is complete, including the
+final call to `config.set()`.
+
+```js
+// karma.conf.js
+const getAsyncConfig = require('./getAsyncConfig.js');
+
+module.exports = async (config) => {
+  const asyncInfo = await getAsyncConfig()
+  config.set({
+    basePath: '../..',
+    frameworks: ['jasmine'],
+    // ... other Karma configuration
+    // ... including the use of `asyncInfo`
+  });
+}
+```
+
+Just as with a synchronous configuration's return value not being used, an
+asynchronous configuration's resolved value will not be used. With both
+configuration types, `config.set()` MUST BE called to assign the configuration
+object.
+
+If you are using the `parseConfig` method from the public API, then please see
+the [Public API Documentation][dev/public-api] for details, usage, and
+addtional information.
+
 ## File Patterns
 All of the configuration options, which specify file paths, use the [minimatch][minimatch] library to facilitate flexible
 but concise file expressions so you can easily list all of the files you want to include and exclude.
@@ -526,7 +556,7 @@ The plugin must provide an express/connect middleware function (details about th
 function CustomMiddlewareFactory (config) {
   return function (request, response, /* next */) {
     response.writeHead(200)
-    return response.end("content!")
+    return response.end('content!')
   }
 }
 ```
@@ -890,5 +920,6 @@ If you see this error, you can try increasing the socket connection timeout.
 [config/files]: files.html
 [config/browsers]: browsers.html
 [config/preprocessors]: preprocessors.html
+[dev/public-api]: ../dev/public-api.html
 [log4js]: https://github.com/nomiddlename/log4js-node
 [minimatch]: https://github.com/isaacs/minimatch