Add doc section on volume expansion

docs/README.md

@@ -1754,6 +1754,35 @@ the hieradata configuration file. Refer to section [4.13 hieradata](#413-hierada
 User defined values take precedence over boot generated values in the Magic Castle
 Puppet data hierarchy.
 
+### 10.15 Expand a volume
+
+Volumes defined in the `volumes` map can be expanded at will. After their creation, you can
+increase their size in the `main.tf` then call `terraform apply` and the associated block
+device will be expanded.
+
+To benefit from the new storage, the following commands need to be ran as root
+on the instance to which the expanded volume is attached.
+1. Identify the physical volume path
+    ```
+    pvscan
+    ```
+2. Expand the physical volume
+    ``` 
+    pvresize /dev/vdxyz # replace vdxyz by the volume identify at step 1
+    ```
+3. Identify the volume group path
+    ``` 
+    lvdisplay
+    ```
+4. Expand the volume group using step volume group path identified
+    ```
+    lvextend -l '+100%FREE' -r /dev/project_vg/project
+    ```
+5. Resize the XFS fileystem:
+    ```
+    xfs_growfs /dev/project_vg/project
+    ```
+
 ## 11. Customize Magic Castle Terraform Files
 
 You can modify the Terraform module files in the folder named after your cloud