Re-added the warning about the .git directory.

The re added version warns to not copy it instead of recommending it's
deletion.

docs/existing-project.md

@@ -4,7 +4,8 @@ It's also possible to use Symfony Docker with existing projects!
 
 First, [download this skeleton](https://github.com/dunglas/symfony-docker).
 
-If you cloned the Git repository you can copy the files from the repository using git and tar:
+If you cloned the Git repository, be sure to not copy the `.git` directory to prevent conflicts with the `.git` directory already in your existing project.
+You can copy the contents of the repository using git and tar. This will not contain `.git` or any uncommited changes.
 
     git archive --format=tar HEAD | tar -xC my-existing-project/