Added known boot modes

README.md

@@ -1,3 +1,14 @@
 # MD398 Reverse Engineering
 
 Resources for the Tytera (TYT) MD398 (aka Radioddity GD-55).
+
+## Boot modes
+
+Different boot modes seem to be available:
+
+
+| PTT | TOP SIDE BUTTON | BOTTOM SIDE BUTTON | EMERGENCY BUTTON | Function |
+|:---:|:---------------:|:------------------:|:----------------:| -------- |
+| *X* |                 | *X* |    | Flip + Mirror screen | 
+|     | *X*             | *X* |    | ??? Black screen no chime, upgrade mode? |
+|     |                 | *X* | *X* | GPS Test mode |