Update README.md

Author: liruilong940607

README.md

@@ -32,6 +32,8 @@ Using NerfAcc,
 
 ## Installation
 
+**Dependence**: Please install [Pytorch](https://pytorch.org/get-started/locally/) first.
+
 The easist way is to install from PyPI. In this way it will build the CUDA code **on the first run** (JIT).
 ```
 pip install nerfacc