Improve the suggested rsync command for updates

When migrating or updating an existing project, it is important not to
overwrite the git files.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

README.md

@@ -71,7 +71,7 @@ the files in your existing project by using `rsync` or similar tools:
 ```sh
 cd /tmp
 cookiecutter gh:frequenz-floss/frequenz-repo-config-python --directory=cookiecutter
-rsync -r new-project/ /path/to/existing/project
+rsync -vr --exclude=.git/ new-project/ /path/to/existing/project
 cd /path/to/existing/project
 git diff
 # Fix all the `TODO`s and cleanup the generated files
@@ -83,6 +83,9 @@ git commit -a
     The trailing slash in `new-project/` and the lack of it in
     `/path/to/existing/project` are meaningful to `rsync`.
 
+    Also make sure to **exclude** the `.git/` directory to avoid messing up
+    with your local git repository.
+
 ## Update an existing project
 
 To update an existing project you can use the [Cookiecutter *replay