pkg/doc.txt: document the package source code locations
pkg/doc.txt: document the package source code locations

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