Add Mentum to ADOPTERS.md
- [Lendico](https://www.lendico.de/)
- [M1 Finance](https://www.m1finance.com/)
- [MattePaint](https://www.mattepaint.com/)
+- [Mentum](https://mentumqr.com/)
- [MercedesBenz.io](https://www.mercedes-benz.io/)
- [Mulligan Funding](https://www.mulliganfunding.com/)
- [Mythical Games](https://mythicalgames.com/)
