Add 'volume prune' to the volume commands index
@@ -106,6 +106,7 @@ read the [`dockerd`](dockerd.md) reference page.
| [volume create](volume_create.md) | Creates a new volume where containers can consume and store data |
| [volume inspect](volume_inspect.md) | Display information about a volume |
| [volume ls](volume_ls.md) | Lists all the volumes Docker knows about |
+| [volume prune](volume_prune.md) | Remove all unused volumes |
| [volume rm](volume_rm.md) | Remove one or more volumes |
