provide clarifying example of onStdOut

Author: thescientist13

README.md

@@ -130,7 +130,26 @@ runner.runCommand(
 );
 ```
 
-`onStdOut` - a callback function that is invoked with a string each time the child process writes to std out. Defaults to `null`.
+##### onStdOut
+This is a callback function that is invoked with a string each time the child process writes to std out. Defaults to `null`.  In conjunction with `stopCommand`, this can be useful to listen for when a long running command is "ready", like waiting for a web server to start.
+
+```js
+before(async function () {
+  await runner.setup(outputPath);
+
+  await new Promise((resolve, reject) => {
+    runner.runCommand(
+      '/path/to/cli.js',
+      "develop",
+      { onStdOut: (message) => {
+        if (message.includes(`Started local development server at http://localhost:8080`)) {
+          resolve();
+        }
+      }}
+    ).catch(reject)
+  });
+});
+```
 
 ### Runner.teardown