Add format to docker stack ls

Signed-off-by: Boaz Shuster <ripcurld.github@gmail.com>

docs/reference/commandline/stack_ls.md

@@ -24,7 +24,8 @@ Aliases:
   ls, list
 
 Options:
-      --help   Print usage
+      --help            Print usage
+      --format string   Pretty-print stacks using a Go template
 ```
 
 ## Description
@@ -43,6 +44,30 @@ vossibility-stack  6
 myapp              2
 ```
 
+### Formatting
+
+The formatting option (`--format`) pretty-prints stacks using a Go template.
+
+Valid placeholders for the Go template are listed below:
+
+| Placeholder | Description        |
+| ----------- | ------------------ |
+| `.Name`     | Stack name         |
+| `.Services` | Number of services |
+
+When using the `--format` option, the `stack ls` command either outputs
+the data exactly as the template declares or, when using the
+`table` directive, includes column headers as well.
+
+The following example uses a template without headers and outputs the
+`Name` and `Services` entries separated by a colon for all stacks:
+
+```bash
+$ docker stack ls --format "{{.Name}}: {{.Services}}"
+web-server: 1
+web-cache: 4
+```
+
 ## Related commands
 
 * [stack deploy](stack_deploy.md)