add en docs for gan (#1894)

* add en docs for gan

* update

* update

* update index

doc/fluid/user_guides/cv_case/index_en.rst

@@ -6,5 +6,6 @@ Computer Vision
 ..  toctree::
     :titlesonly:
 
+    image_classification/README.md
     gan/README.md
 

doc/fluid/user_guides/index_en.rst

@@ -19,6 +19,8 @@ Overview
    
     - `Simple Case <../user_guides/simple_case/index_en.html>`_ ：introduces basic cases of Paddle
 
+    - `Computer Vision <../user_guides/cv_case/index_en.html>`_ ：introduces cases of using paddle to realize Computer Vision task
+
     - `Natural Language Processing <../user_guides/nlp_case/index_en.html>`_：introduces cases of using paddle to realize Natural Language Processing tasks
 
     - `Recommend <../user_guides/rec_case/index_en.html>`_：introduces cases of using paddle to realize Recommend tasks
@@ -29,6 +31,7 @@ Overview
     :hidden:
 
     simple_case/index_en.rst
+    cv_case/index_en.rst
     nlp_case/index_en.rst
     rec_case/index_en.rst
 	models/index_cn.rst