docs(backups): describes file hierarchy with VHD directories (#6337)

@xen-orchestra/backups/docs/VM backups/README.md

@@ -14,6 +14,8 @@
 
 ## File structure on remote
 
+### with vhd files
+
 ```
 <remote>
 └─ xo-vm-backups
@@ -30,6 +32,19 @@
      └─ <YYYYMMDD>T<HHmmss>.xva.checksum
 ```
 
+### with vhd directories
+
+When `useVhdDirectory` is enabled on the remote, the directory containing the VHDs has a slightly different architecture:
+
+```
+<vdis>/<job UUID>/<VDI UUID>
+  ├─ <YYYYMMDD>T<HHmmss>.alias.vhd // contains the relative path to a VHD directory
+  ├─ <YYYYMMDD>T<HHmmss>.alias.vhd
+  └─ data
+    ├─ <uuid>.vhd // VHD directory format is described in vhd-lib/Vhd/VhdDirectory.js
+    └─ <uuid>.vhd
+```
+
 ## Attributes
 
 ### Of created snapshots