refactor directory for fluid NMT models.

fluid/neural_machine_translation/README.md

+The minimum PaddlePaddle version needed for the code sample in this directory is the lastest develop branch. If you are on a version of PaddlePaddle earlier than this, [please update your installation](http://www.paddlepaddle.org/docs/develop/documentation/en/build_and_install/pip_install_en.html).
+
+---
+
+This is a collection of example models for neural machine translation and neural sequence modeling.
+
+### TODO
+
+This project is still under active development.