add alias description

Signed-off-by: Andreas Hechenberger <world@greenstation.de>

CONTRIBUTING.md

@@ -50,7 +50,9 @@ If you can certify it, then just add a line to every git commit message:
 
 Use your real name (sorry, no pseudonyms or anonymous contributions).
 If you set your `user.name` and `user.email` git configs, you can sign your
-commit automatically with `git commit -s`.
+commit automatically with `git commit -s`. You can also use git [aliases](https://git-scm.com/book/tr/v2/Git-Basics-Git-Aliases)
+like `git config --global alias.ci 'commit -s'`. Now you can commit with
+`git ci` and the commit will be signed.
 
 ### Apply a license