Update README.md (#4)

Add a comment ‘cd Multimodal-GPT' to help users enter the Multimodal-GPT project.

README.md

@@ -18,6 +18,7 @@ To install the package in an existing environment, run
 
 ```bash
 git clone https://github.com/open-mmlab/Multimodal-GPT.git
+cd Multimodal-GPT
 pip install -r requirements.txt
 pip install -e. -v
 ```