README: add more badges
README: add more badges

The README document was missing several relevant badges/links:

- Latest release
- License info
- API docs
- Wiki
- StackOverflow
- Twitter

The addition of these links will, hopefully, help people
quickly navigate the broader ecosystem.

Additionally, all badges (except for the CI badge) now
conform to roughly the same style.

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