cmake --workflow: print usage message

Source/cmakemain.cxx

@@ -956,7 +956,14 @@ int do_workflow(int ac, char const* const* av)
   }
 
   if (presetName.empty() && !listPresets) {
-    std::cerr << "TODO: Usage\n";
+    /* clang-format off */
+    std::cerr <<
+      "Usage: cmake --workflow [options]\n"
+      "Options:\n"
+      "  --preset <preset> = Workflow preset to execute.\n"
+      "  --list-presets    = List available workflow presets.\n"
+      ;
+    /* clang-format on */
     return 1;
   }