update README

README.md

@@ -12,9 +12,18 @@ accepts call to execute in gulp pipeline.
 ```javascript
 var gulp = require("gulp");
 var gulpFunction = require("gulp-function");
+var Q = require("q");
 
 var myFunction = function () {
+    var done = Q.defer();
     console.log("Hello World!")
+    
+    // NOTE:
+    // you can use done.resolve as callback function
+    // of any async tasks within this function
+    done.resolve();
+    
+    return done.promise;
 }
 
 gulp.task('gulpTest',function() {
@@ -25,4 +34,5 @@ gulp.task('gulpTest',function() {
 ```
 
 > Note: The first argument of gulpFunction can also be an array of multiple functionnames.
+Each function can return a promise. the pipe stop will finish when every promise is fullfilled.  
 > Note: the second argument can be empty (defaults to 'forEach') or 'atEnd'