Message about using --parallel, and how other CWL runners are availab…

…le. (#1671)

Add message about using --parallel, and how other CWL runners are available.

cwltool/executors.py

@@ -191,6 +191,15 @@ def check_for_abstract_op(tool: CWLObjectType) -> None:
             return (self.final_output[0], self.final_status[0])
         return (None, "permanentFail")
 
+    def message_about_other_runners(self) -> None:
+        _logger.info(
+            "Need to grow beyond `cwltool` and scale up your workflows to run on a multi-node cluster, or in the cloud?"
+        )
+        _logger.info(
+            "CWL workflows are portable and run on many commercial and open source platforms."
+        )
+        _logger.info("Visit https://www.commonwl.org/implementations/ to learn more.")
+
 
 class SingleJobExecutor(JobExecutor):
     """Default single-threaded CWL reference executor."""
@@ -203,6 +212,10 @@ def run_jobs(
         runtime_context: RuntimeContext,
     ) -> None:
 
+        _logger.info(
+            "Using default serial job executor.  Use `cwltool --parallel` to run multiple steps at a time."
+        )
+
         process_run_id = None  # type: Optional[str]
 
         # define provenance profile for single commandline tool
@@ -252,6 +265,7 @@ def run_jobs(
                 else:
                     logger.error("Workflow cannot make any more progress.")
                     break
+            self.message_about_other_runners()
         except (
             ValidationException,
             WorkflowException,
@@ -416,6 +430,10 @@ def run_jobs(
         runtime_context: RuntimeContext,
     ) -> None:
 
+        _logger.info(
+            "Using parallel job executor.  Multiple steps will run at once as hardware resources permit."
+        )
+
         self.taskqueue = TaskQueue(
             threading.Lock(), psutil.cpu_count()
         )  # type: TaskQueue
@@ -456,6 +474,7 @@ def run_jobs(
         finally:
             self.taskqueue.drain()
             self.taskqueue.join()
+        self.message_about_other_runners()
 
 
 class NoopJobExecutor(JobExecutor):