doc: add link to forum on the main page
doc: add link to forum on the main page

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