[doc] Add "Know issues" about file not uploaded when no change on entity

Related to dustin10#123, dustin10#8, ...

Resources/doc/index.md

@@ -429,3 +429,37 @@ vich_uploader:
 store files. The bundle ships with vich_uploader.storage.file_system
 and vich_uploader.storage.gaufrette see
 [FileSystemStorage VS GaufretteStorage](#filesystemstorage-vs-gaufrettestorage)
+
+## Know issues
+
+### The file is not updated if there are not other changes in the entity
+As the bundle is listen on Doctrine event `prePersist` and `preUpdate`, which are not fired
+when there is no change on field mapped by Doctrine, the file upload is not handled.
+
+A workaround to solve this issue is to manually generate a change:
+
+```
+class Product
+{
+    // ...
+    
+    /**
+     * @ORM\Column(type="datetime")
+     *
+     * @var \DateTime $updatedAt
+     */
+    protected $updatedAt;
+    
+    // ...
+    
+    public function setImage($image)
+    {
+        $this->image = $image;
+        
+        if ($this->image) {
+            $this->updatedAt = new \DateTime('now');
+        }
+    }
+}
+```
+See issue [GH-123](https://github.com/dustin10/VichUploaderBundle/issues/123)