Add ninja to dependency (#21)

Author: WoosukKwon

README.md

@@ -6,7 +6,8 @@
 pip install psutil numpy ray torch
 pip install git+https://github.com/huggingface/transformers  # Required for LLaMA.
 pip install sentencepiece  # Required for LlamaTokenizer.
-pip install flash-attn  # This may take up to 20 mins.
+pip install ninja  # To parallelize the compilation of flash-attn.
+pip install flash-attn  # This may take up to 10 mins.
 pip install -e .
 ```