add flags file (#898)

* add flag files

* add flags file

* delete space

doc/fluid/api/index_en.rst

@@ -5,6 +5,7 @@ API Reference
 ..  toctree::
     :maxdepth: 1
 
+    ../flags_en.rst
     ../api_guides/index_en.rst
     fluid.rst
     average.rst

doc/fluid/api_cn/index_cn.rst

@@ -5,6 +5,7 @@ API
 ..  toctree::
     :maxdepth: 1
 
+    ../flags_cn.rst
     ../api_guides/index_cn.rst
     fluid_cn.rst
     average_cn.rst