README: replace StackOverflow badge
README: replace StackOverflow badge

The current StackOverflow badge sometimes shows as 'invalid'.
This is, of course, not really wanted.

Replace it by one that is a bit more reliable.

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