Added further explanation of the -707 error code (#691)

src/TypeDef.h

@@ -387,6 +387,11 @@
 
 /*!
   \brief SX126x failed to execute SPI command.
+  Often this means that the module is trying to use TCXO while
+  XTAL is connected (or vice versa). Make sure your crystal setup
+  (e.g. TCXO reference voltage) matches your hardware by setting
+  "tcxoVoltage" to 0 when using XTAL module, or to appropriate value
+  when using TCXO module.
 */
 #define RADIOLIB_ERR_SPI_CMD_FAILED                            (-707)