add critical fault for brownout

docs/docs.md

@@ -699,6 +699,7 @@ and we'll try to help.
 | NoAXP192 | 6 | This board was expected to have a power management chip and it is missing or broken |
 | InvalidRadioSetting | 7 | The channel tried to set a radio setting which is not supported by this chipset, radio comms settings are now undefined. |
 | TransmitFailed | 8 | Radio transmit hardware failure. We sent data to the radio chip, but it didn't reply with an interrupt. |
+| Brownout | 9 | We detected that the main CPU voltage dropped below the minumum acceptable value |
 
 
 

mesh.proto

@@ -549,6 +549,9 @@ enum CriticalErrorCode {
    */
   TransmitFailed = 8;
 
+  /* We detected that the main CPU voltage dropped below the minumum acceptable value */
+  Brownout = 9;
+
 }
 
 /*