docs: add link to gemini cli installation instructions. (#3)

Author: mctavish

README.md

@@ -10,6 +10,9 @@ Install the Code Review extension by running the following command from your ter
 gemini extensions install https://github.com/gemini-cli-extensions/code-review
 ```
 
+If you do not yet have Gemini CLI installed, or if the installed version is older than 0.4.0, see
+[Gemni CLI installation instructions](https://github.com/google-gemini/gemini-cli?tab=readme-ov-file#-installation).
+
 ## Use the extension
 
 The Code Review extension adds the `/code-review` command to Gemini CLI which analyzes code changes on your current branch for quality issues.