Add comment to the default value

Author: fthomas

modules/core/src/main/resources/default.scala-steward.conf

@@ -4,6 +4,9 @@
 //   Changes to this file are therefore immediately visible to all
 //   Scala Steward instances.
 
+// Repos whose last commit is older than this threshold are considered
+// inactive and are ignored. This setting can be overridden with a greater
+// value in your own repo config file.
 inactivityThreshold = "270 days"
 
 postUpdateHooks = [