Add promises example

README.md

@@ -25,7 +25,14 @@ psd.parse();
 
 console.log(psd.tree().export());
 console.log(psd.tree().childrenAtPath('A/B/C')[0].export());
-psd.image.saveAsPng('./output.png').then(function () { console.log('success!') });
+
+// You can also use promises syntax for opening and parsing
+PSD.open("path/to/file.psd").then(function (psd) {
+  return psd.image.saveAsPng('./output.png');
+}).then(function () {
+  console.log("Finished!");
+});
+
 ```
 
 ### Browser