@GertSanders it is so small indeed, soldering with my soldering iron would be a problem. Maybe a 'secret' connector for this device would help @Koresh, just mount it if you need it...
@MiKa
Hi.
oki, in case i'll add some explanation.. I'll be busy for this week but you can simply use the serial gw example from MySensors for getting started, this should work. I didn't need to create a special board in arduino for this board, so if you're using it, you'll need to install&use the mattairtech SAMD21 board definition for the pin mapping.
https://github.com/mattairtech/ArduinoCore-samd