Add an acceptance test and docs for the down subcommand

Signed-off-by: Daniel Nephin <dnephin@docker.com>

docs/reference/down.md

@@ -0,0 +1,26 @@
+<!--[metadata]>
++++
+title = "down"
+description = "down"
+keywords = ["fig, composition, compose, docker, orchestration, cli,  down"]
+[menu.main]
+identifier="build.compose"
+parent = "smn_compose_cli"
++++
+<![end-metadata]-->
+
+# down
+
+```
+Stop containers and remove containers, networks, volumes, and images
+created by `up`. Only containers and networks are removed by default.
+
+Usage: down [options]
+
+Options:
+    --rmi type      Remove images, type may be one of: 'all' to remove
+                    all images, or 'local' to remove only images that
+                    don't have an custom name set by the `image` field
+    -v, --volumes   Remove data volumes
+
+```