Update ADOPTERS.md
Add Nexxiot as adopter.
- [Mulligan Funding](https://www.mulliganfunding.com/)
- [Mythical Games](https://mythicalgames.com/)
- [National Information Solutions Cooperative (NISC)](https://nisc.coop/)
+- [Nexxiot](https://www.nexxiot.com)
- [Nordstrom](https://nordstrom.com/)
- [Novolabs](https://novolabs.com)
- [Paybase](https://paybase.io/)
