CONTRIBUTING: add example commit
CONTRIBUTING: add example commit

Add an example commit to the *commit conventions* section.
Refer to this in the pull request steps.

ant9000/RIOT,miri64/RIOT,miri64/RIOT,jasonatran/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,miri64/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,authmillenon/RIOT,RIOT-OS/RIOT,OTAkeys/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT,kaspar030/RIOT,authmillenon/RIOT,OTAkeys/RIOT,OlegHahm/RIOT,ant9000/RIOT,ant9000/RIOT,authmillenon/RIOT,OTAkeys/RIOT,jasonatran/RIOT,miri64/RIOT,authmillenon/RIOT,OlegHahm/RIOT,jasonatran/RIOT,kYc0o/RIOT,kYc0o/RIOT,OlegHahm/RIOT,kaspar030/RIOT,ant9000/RIOT,kaspar030/RIOT,kYc0o/RIOT,kYc0o/RIOT,kaspar030/RIOT,jasonatran/RIOT,ant9000/RIOT,authmillenon/RIOT,authmillenon/RIOT,OTAkeys/RIOT,miri64/RIOT,kYc0o/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,OlegHahm/RIOT