Update README.md

Author: dongjun-Lee

README.md

@@ -20,7 +20,7 @@ Used [BahdanauAttention](https://www.tensorflow.org/api_docs/python/tf/contrib/s
 
 ## Requirements
 - Python 3
-- Tensorflow
+- Tensorflow (>=1.8.0)
 - pip install -r requirements.txt
 
 
@@ -117,4 +117,4 @@ To test with pre-trained model, download [pre_trained.zip](https://drive.google.
 ```
 $ unzip pre_trained.zip
 $ python test.py
-```
+```