Update index_en.rst

fix https://github.com/PaddlePaddle/Paddle/issues/8921

doc/v2/faq/index_en.rst

 FAQ
 ====
 
- 
+This document provides frequently asked questions of PaddlePaddle. If your questions are not here, please go to `PaddlePaddle Community <https://github.com/PaddlePaddle/Paddle/issues>`_to find answers or open an `issue <https://github.com/PaddlePaddle/Paddle/issues/new>`_ , we will reply in time.
+
 ..  toctree::
   :maxdepth: 1