Print a usage description when the user enters the swift-format comma…

…nd without arguments

Sources/swift-format/Subcommands/Format.swift

@@ -36,6 +36,16 @@ extension SwiftFormatCommand {
     var performanceMeasurementOptions: PerformanceMeasurementsOptions
 
     func validate() throws {
+      if formatOptions.paths.isEmpty && !inPlace {
+        throw ValidationError(
+          """
+          No input files specified. Use one of the following:
+            - Provide the path to a directory along with the '--recursive' option to format all Swift files within it.
+            - Provide the path to a specific Swift source code file.
+          And use '--in-place' to overwrite files in-place.
+          """
+        )
+      }
       if inPlace && formatOptions.paths.isEmpty {
         throw ValidationError("'--in-place' is only valid when formatting files")
       }