Add instructions for interactive mode

Add instructions on how to use psdk interactive mode to the REAME.md
file.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>

README.md

@@ -11,7 +11,7 @@ one SDK to store:
 The background information about the design of psdk can be found at:
 	https://designs.apertis.org/private/latest/maintaining-workspace-across-sdk-updates.html
 
-psdk has the following options:
+Calling psdk without arguments will start the interactive mode. However it has the following options:
 
     user@apertis:~$ psdk -h
     usage: psdk [-h] [-e] [-s] [-c] [-i] [path]
@@ -57,6 +57,57 @@ SDK customization to the new version.
 psdk automates the tasks of moving the developer customization to a second
 disk and configuring the SDK to use the second disk.
 
+## Upgrading to a new SDK using psdk interactive mode
+
+First step is to prepare the persistent disk on the old SDK. Start by adding a
+second disk to the old SDK. This disk should be big enough to host the contents
+of /home and /opt: We recommend using more than 40GiB and no less than 20GiB.
+As we will use dynamically allocated VDI images the unused space does not
+consume disk space on the host, so if in doubt, create a larger disk. Step by
+step instructions of how to add a second disk are on the section *Upgrading to
+a new SDK using psdk*.
+
+After adding the second disk, start the old SDK and click on the Desktop icon
+*Persistent Disk Setup*. psdk will open in interactive mode, and  it will show:
+
+    Persistent disk setup
+    
+    For help visit:
+    	https://developer.apertis.org/latest/sdk-usage.html
+    
+    Status:
+    	Empty disk found: /dev/sdb
+    
+    Do you want to initialize the empty disk /dev/sdb and configure the SDK to use it as persistent disk?
+    (yes and press enter):
+
+Typing *yes* and pressing enter will initialize the disk, will configure the
+SDK, and will reboot the SDK to apply the changes.
+
+Turn off the old SDK.
+
+Now on the new SDK add the persistent disk you configured on the old SDK.
+Detailed instructions on how to do this are on step 5 of section *Upgrading to
+a new SDK using psdk*.
+
+Start the new SDK and click on the Desktop icon *Persistent Disk Setup*. psdk
+will open in interactive mode, and  it will show:
+
+    Persistent disk setup
+    
+    For help visit:
+    	https://developer.apertis.org/latest/sdk-usage.html
+    
+    Status:
+    	Persistent disk in use: False
+    	Persistent disk found: /dev/sdb1
+    
+    Do you want to configure the SDK to use the persistent disk /dev/sdb1?
+    (yes and press enter):
+    
+Typing *yes* and pressing enter will configure the SDK to use the persistent
+disk and will reboot the SDK to apply the changes.
+
 ## Upgrading to a new SDK using psdk (short version)
 - On the old SDK:
   - Add a new second disk to be used as persistent storage