Add markdown for man page of docker plugin ls

This fix adds markdown for man page of `docker plugin ls`,
based on https://github.com/docker/docker/pull/28627#issuecomment-276731752

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

man/src/plugin/ls.md

@@ -0,0 +1,43 @@
+Lists all the plugins that are currently installed. You can install plugins
+using the `docker plugin install` command.
+You can also filter using the `-f` or `--filter` flag.
+
+## Filters
+
+Filter output based on these conditions:
+   - enabled=(true|false) - plugins that are enabled or not
+   - capability=<string> - filters plugins based on capabilities (currently `volumedriver`, `networkdriver`, `ipamdriver`, or `authz`)
+
+## Format
+
+   Pretty-print plugins using a Go template.
+   Valid placeholders:
+      .ID - Plugin ID.
+      .Name - Plugin Name.
+      .Description - Plugin description.
+      .Enabled - Whether plugin is enabled or not.
+
+# EXAMPLES
+## Display all plugins
+
+    $ docker plugin ls
+    ID                  NAME                                    DESCRIPTION                         ENABLED
+    869080b57404        tiborvass/sample-volume-plugin:latest   A sample volume plugin for Docker   true
+    141bf6c02ddd        vieux/sshfs:latest                      sshFS plugin for Docker             false
+
+## Display plugins with their ID and names
+
+    $ docker plugin ls --format "{{.ID}}: {{.Name}}"
+    869080b57404: tiborvass/sample-volume-plugin:latest
+
+## Display enabled plugins
+
+    $ docker plugin ls --filter enabled=true
+    ID                  NAME                                    DESCRIPTION                         ENABLED
+    869080b57404        tiborvass/sample-volume-plugin:latest   A sample volume plugin for Docker   true
+
+## Display plguins with `volumedriver` capability
+
+    $ docker plugin ls --filter capability=volumedriver --format "table {{.ID}}\t{{.Name}}"
+    ID                  Name
+    869080b57404        tiborvass/sample-volume-plugin:latest