Add documentation on the "managed" volume parameter

Author: cmd-ntrf

docs/README.md

@@ -632,6 +632,10 @@ Each volume in map is defined a key corresponding to its and a map of attributes
   - GCP: `pd-ssd`
   - OpenStack: `null`
   - OVH: `null`
+- `managed` (optional, def: `true`): set to `false` to indicate that the volume already
+exists and should not be created by Terraform only mounted. When a volume is configured with `managed = false`,
+its name needs to match the following pattern `{tag}-{name}-{mounting_hostname}`,
+for example `nfs-home-mgmt1`.
 
 Volumes with a tag that have no corresponding instance will not be created.