Add link to firmware repo

_index.md

@@ -47,6 +47,7 @@ We moved from our [Hamburg GitLab](https://gitlab.hamburg.ccc.de/card10/) to:
 **!Caution!** Card10 is still in the prototyping phase, bear in mind that hardware and PCB layout changes may still occur.
 
 - [Hardware](http://git.card10.badge.events.ccc.de/card10/hardware)
+- [Firmware](http://git.card10.badge.events.ccc.de/card10/firmware)
 - [LogBook](/en/logbook)
 - [Interhacktions](/en/interhacktions)
 - [Fundamental board](/en/fundamental-board-overview)