docs: simplify commit instruction (#346)

prefer `git commit` without `-m` as it forces you to think about the
commit message. `git add .` is also a bit dangerous since you can
accidentally add files you did not mean to add. `git commit -a` rather
only adds changes tracked files.

CONTRIBUTING.md

@@ -67,8 +67,7 @@ git checkout -b <branch_name>
 And start making your changes! Once you're done, you can commit your changes and push them to your forked repo.
 
 ```bash
-git add .
-git commit -m 'your commit message'
+git commit -a
 git push <your_forked_github>
 ```