doc(Makefile): add comment to check deis/workflow-e2e

Author: arschles

Makefile

@@ -94,5 +94,7 @@ test-unit:
 test-functional:
 	@echo "Implement functional tests in _tests directory"
 
+test-integration:
+	@echo "Check https://github.com/deis/workflow-e2e for the complete interation test suite"
 
 .PHONY: build clean commit-hook full-clean postgres setup-venv test test-style test-unit test-functional