Update README.md to add megablocks requirement for mixtral (#2033)

Author: 0-hero

README.md

@@ -72,6 +72,10 @@ Install vLLM with pip or [from source](https://vllm.readthedocs.io/en/latest/get
 ```bash
 pip install vllm
 ```
+**NOTE:** The Mixtral model additionally requires `megablocks` which can be installed with pip or [from source](https://github.com/stanford-futuredata/megablocks) on **Python 3.10**:
+```bash
+pip install megablocks
+```
 
 ## Getting Started