Add link to feature flags docs (open-telemetry#1523)

Co-authored-by: Austin Parker <austin@ap2.io>

Author: curusarn

Makefile

@@ -131,6 +131,7 @@ start:
 	@echo "Go to http://localhost:8080/jaeger/ui for the Jaeger UI."
 	@echo "Go to http://localhost:8080/grafana/ for the Grafana UI."
 	@echo "Go to http://localhost:8080/loadgen/ for the Load Generator UI."
+	@echo "Go to https://opentelemetry.io/docs/demo/feature-flags/ to learn how to change feature flags."
 
 .PHONY: start-minimal
 start-minimal:
@@ -141,6 +142,7 @@ start-minimal:
 	@echo "Go to http://localhost:8080/jaeger/ui for the Jaeger UI."
 	@echo "Go to http://localhost:8080/grafana/ for the Grafana UI."
 	@echo "Go to http://localhost:8080/loadgen/ for the Load Generator UI."
+	@echo "Go to https://opentelemetry.io/docs/demo/feature-flags/ to learn how to change feature flags."
 
 # Observabilty-Driven Development (ODD)
 .PHONY: start-odd
@@ -153,6 +155,7 @@ start-odd:
 	@echo "Go to http://localhost:8080/grafana/ for the Grafana UI."
 	@echo "Go to http://localhost:8080/loadgen/ for the Load Generator UI."
 	@echo "Go to http://localhost:11633/ for the Tracetest Web UI."
+	@echo "Go to https://opentelemetry.io/docs/demo/feature-flags/ to learn how to change feature flags."
 
 .PHONY: stop
 stop: