docs(readme): use axe.run() promise API (#2072)

* docs(readme): use axe.run() promise API

* Update README.md

README.md

@@ -50,12 +50,15 @@ Now include the javascript file in each of your iframes in your fixtures or test
 Now insert calls at each point in your tests where a new piece of UI becomes visible or exposed:
 
 ```js
-axe.run(function (err, results) {
-  if (err) throw err;
-    ok(results.violations.length === 0, 'Should be no accessibility issues');
-    // complete the async call
-    ...
-});
+axe.run()
+  .then(results => {
+    if (results.violations.length) {
+      throw new Error('Accessibility issues found')
+    }
+  })
+  .catch(err => {
+    console.error('Something bad happened:', err.message)
+  })
 ```
 
 ## Supported Browsers