Fix uploadValidator example

Previous code sample had race condition; see #62.

Added note that the "saved" event can be used to move the file.

README.md

@@ -460,15 +460,12 @@ Can be overridden to enable async validation and preparing.
 
 ```javascript
 uploader.uploadValidator = function(event, callback){
-    fs.mkdtemp('/tmp/foo-', function(err, folder) {
-        if (err) {
-            callback( false ); // abort
-        }
-        else {
-            uploader.dir = folder;
-            callback( true ); // ready
-        }
-    });
+    // asynchronous operations allowed here; when done,
+    if (/* success */) {
+        callback(true);
+    } else {
+        callback(false);
+    }
 };
 ```
 
@@ -514,6 +511,8 @@ The transmission of a file is complete.
 
 A file has been saved.  It is recommended that you check `event.file.success` to tell whether or not the file was saved without errors.
 
+In this event, you can safely move the saved file to a new location.
+
 ##### Event Properties
 
 * `event.file` The same file object that would have been passed during the `start` event earlier.