Update README.md:Git panel is not visible (#891)

* Update README.md:Git panel is not visible

Even if the package is installed, it does not work unless it is in the uninstalled packages list. It is caused by a bug in jupyterlab according to this issue jupyterlab/jupyterlab#8122

* Prefer `jupyter lab clean --all`

Co-authored-by: Frédéric Collonval <[email protected]>

Author: EnisBerk

README.md

@@ -137,7 +137,10 @@ If they do not match or one is missing, please [reinstall the package](README.md
     ```bash
     jupyter labextension install @jupyterlab/git
     ```
-
+    
+    If you see `@jupyterlab/git`, your installation may have been corrupted. You can run `jupyter lab clean --all` and
+    reinstall all your extensions.
+    
 ## Contributing
 
 If you would like to contribute to the project, please read our [contributor documentation](https://github.com/jupyterlab/jupyterlab/blob/master/CONTRIBUTING.md).