Add requirements link for NetworkMorphism (#1272)

* Update BuiltinTuner.md

* Update BuiltinTuner.md

* Update BuiltinTuner.md

docs/en_US/BuiltinTuner.md

@@ -263,7 +263,7 @@ advisor:
 
 **Installation**
 
-NetworkMorphism requires [PyTorch](https://pytorch.org/get-started/locally), so users should install it first.
+NetworkMorphism requires [PyTorch](https://pytorch.org/get-started/locally) and [Keras](https://keras.io/#installation), so users should install them first. The corresponding requirements file is [here](https://github.com/microsoft/nni/blob/master/examples/trials/network_morphism/requirements.txt).
 
 **Suggested scenario**