Add network --format flag to ls

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

docs/reference/commandline/network_ls.md

@@ -20,6 +20,7 @@ Aliases:
 
 Options:
   -f, --filter value   Provide filter values (i.e. 'dangling=true') (default [])
+      --format string  Pretty-print networks using a Go template
       --help           Print usage
       --no-trunc       Do not truncate the output
   -q, --quiet          Only display volume names
@@ -169,6 +170,38 @@ $ docker network rm `docker network ls --filter type=custom -q`
 A warning will be issued when trying to remove a network that has containers
 attached.
 
+## Formatting
+
+The formatting options (`--format`) pretty-prints networks output
+using a Go template.
+
+Valid placeholders for the Go template are listed below:
+
+Placeholder | Description
+------------|------------------------------------------------------------------------------------------
+`.ID`       | Network ID 
+`.Name`     | Network name
+`.Driver`   | Network driver
+`.Scope`    | Network scope (local, global)
+`.IPv6`     | Whether IPv6 is enabled on the network or not.
+`.Internal` | Whether the network is internal or not.
+`.Labels`   | All labels assigned to the network.
+`.Label`    | Value of a specific label for this network. For example `{{.Label "project.version"}}`
+
+When using the `--format` option, the `network ls` command will either
+output 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
+`ID` and `Driver` entries separated by a colon for all networks:
+
+```bash
+$ docker network ls --format "{{.ID}}: {{.Driver}}"
+afaaab448eb2: bridge
+d1584f8dc718: host
+391df270dc66: null
+```
+
 ## Related information
 
 * [network disconnect ](network_disconnect.md)