Document local pipeline demo

Makefile

@@ -1,4 +1,16 @@
-.PHONY: demo-ui demo-down demo-logs
+.DEFAULT_GOAL := help
+
+.PHONY: help demo-ui demo-down demo-logs
+
+help:
+	@printf '%s\n' \
+	  'SciMesh developer commands:' \
+	  '  make demo-ui                 Start the local UI pipeline demo with 2 workers.' \
+	  '  make demo-ui WORKERS=3       Start the demo with 3 local workers.' \
+	  '  make demo-logs               Follow coordinator logs for the demo.' \
+	  '  make demo-down               Stop demo containers and workers.' \
+	  '' \
+	  'After make demo-ui: open http://localhost:18080/ui (operator / demo-ui-secret).'
 
 # Convenient entry points from the repository root. Extra settings are passed
 # through, for example: make demo-ui WORKERS=3