CONTRIBUTING: link to both IRC and Matrix
CONTRIBUTING: link to both IRC and Matrix

Link to both the IRC and Matrix end of the chat, as both are used.

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