If serial.log is disabled, log how to enable it

That will hopefully help a bit against the endless stream of "serial
.log was empty" statements on the bug tracker. Or not. Here's hope.

src/octoprint/__init__.py

@@ -216,7 +216,7 @@ def init_logging(settings, use_logging_file=True, logging_file=None, default_con
 			},
 			"loggers": {
 				"SERIAL": {
-					"level": "CRITICAL",
+					"level": "INFO",
 					"handlers": ["serialFile"],
 					"propagate": False
 				},

src/octoprint/printer/standard.py

@@ -208,6 +208,9 @@ class Printer(PrinterInterface, comm.MachineComPrintCallback):
 
 		from octoprint.logging.handlers import SerialLogHandler
 		SerialLogHandler.on_open_connection()
+		if not logging.getLogger("SERIAL").isEnabledFor(logging.DEBUG):
+			# if serial.log is not enabled, log a line to explain that to reduce "serial.log is empty" in tickets...
+			logging.getLogger("SERIAL").info("serial.log is currently not enabled, you can enable it via Settings > Serial Connection > Log communication to serial.log")
 
 		self._comm = comm.MachineCom(port, baudrate, callbackObject=self, printerProfileManager=self._printerProfileManager)