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
+
+```

docs/reference/index.md

@@ -14,10 +14,14 @@ parent = "smn_compose_ref"
 The following pages describe the usage information for the [docker-compose](docker-compose.md) subcommands. You can also see this information by running `docker-compose [SUBCOMMAND] --help` from the command line.
 
 * [build](build.md)
+* [config](config.md)
+* [create](create.md)
+* [down](down.md)
 * [events](events.md)
 * [help](help.md)
 * [kill](kill.md)
 * [logs](logs.md)
+* [pause](pause.md)
 * [port](port.md)
 * [ps](ps.md)
 * [pull](pull.md)
@@ -27,6 +31,7 @@ The following pages describe the usage information for the [docker-compose](dock
 * [scale](scale.md)
 * [start](start.md)
 * [stop](stop.md)
+* [unpause](unpause.md)
 * [up](up.md)
 
 ## Where to go next