Document brightness (-b) option switch.

redshift.1

@@ -26,6 +26,9 @@ Verbose output
 \fB\-V\fR
 Show program version
 .TP
+\fB\-b\fR N
+Screen brightness to apply (max is 1.0)
+.TP
 \fB\-c\fR FILE
 Load settings from specified configuration file
 .TP

src/redshift.c

@@ -312,7 +312,8 @@ print_help(const char *program_name)
 	/* TRANSLATORS: help output 4
 	   `list' must not be translated
 	   no-wrap */
-	fputs(_("  -c FILE\tLoad settings from specified configuration file\n"
+	fputs(_("  -b N\t\tScreen brightness to apply (max is 1.0)\n"
+                "  -c FILE\tLoad settings from specified configuration file\n"
 		"  -g R:G:B\tAdditional gamma correction to apply\n"
 		"  -l LAT:LON\tYour current location\n"
 		"  -l PROVIDER\tSelect provider for automatic"