Update README.md

Adding chainability documentation

Author: leog

README.md

@@ -43,7 +43,7 @@ var gulp = require('gulp'),
     rjs = require('gulp-requirejs');
 
 gulp.task('requirejsBuild', function() {
-    rjs({
+    return rjs({
         baseUrl: 'path/to/your/base/file.js',
         out: 'FILENAME_TO_BE_OUTPUTTED',
         shim: {
@@ -55,6 +55,8 @@ gulp.task('requirejsBuild', function() {
 });
 ```
 
+Note: In order to let gulp know that the optimization completes, return the rjs stream.
+
 ### Error handling
 
 gulp-requirejs will emit errors when you don't pass an options object and if the `baseUrl` or `out` properties are undefined.