Merge pull request #1738 from infosiftr/swarm-deprecated

Add "Classic Swarm" deprecation notice

swarm/deprecated.md

@@ -0,0 +1,3 @@
+> Classic Swarm has been archived and is no longer actively developed. You may want to use the Swarm mode built into the Docker Engine instead, or another orchestration system.
+
+(https://github.com/docker/classicswarm#readme)