add link to thumb combo files in readme

readme.md

@@ -7,10 +7,10 @@ English/French/Programming.
 
 Since the Corne (36 keys) and Urchin (34 keys) have similar layouts, both
 keyboards use the same configuration. The only exception is the combos using
-thumb keys, which are in their own files (combo_34 and combos_36). Keyboards
-also have their own keymap files, but in pratice they use the same mappings.
-This repos uses Github Actions to generate both firmwares simultaneously on each
-push.
+thumb keys, which are in their own files ([combos_34](behaviors/combos_34.dtsi)
+and [combos_36](behaviors/combos_36.dsti)). Keyboards also have their own keymap
+files, but in pratice they use the same mappings. This repos uses Github Actions
+to generate both firmwares simultaneously on each push.
 
 ![My Corne 36 keys keyboard](images/corne36.jpg)