add readme of /doc

in order to announce readers and developers that the documentation source of PaddlePaddle.org has been moved to Fluiddoc repo

doc/README.md

+# For Readers and Developers
+
+Thanks for reading PaddlePaddle documentation. 
+
+Since **September 17th, 2018**, the **0.15.0 and develop** documentation source has been moved to [Fluiddoc Repo](https://github.com/PaddlePaddle/Paddle).
+
+Please turn to Fluiddoc Repo for the latest documentation.