mention that all undervolt voltage must be negative in config file (fix #171)

etc/lenovo_fix.conf

@@ -44,6 +44,7 @@ cTDP: 0
 # Disable BDPROCHOT (EXPERIMENTAL)
 Disable_BDPROCHOT: False
 
+# All voltage values are expressed in mV and *MUST* be negative (i.e. undervolt)! 
 [UNDERVOLT.BATTERY]
 # CPU core voltage offset (mV)
 CORE: 0
@@ -56,6 +57,7 @@ UNCORE: 0
 # Analog I/O voltage offset (mV)
 ANALOGIO: 0
 
+# All voltage values are expressed in mV and *MUST* be negative (i.e. undervolt)!
 [UNDERVOLT.AC]
 # CPU core voltage offset (mV)
 CORE: 0