doc: mention lnd-mode when upgrading from old version

Users updating from a pre-0.3.0 version need to add the lnd-mode flag
and set it to "integrated" if they want to continue running LiT the same
way as they did before. This was forgotten to be mentioned in the
upgrade doc.

Author: guggero

doc/config-lnd-integrated.md

@@ -149,6 +149,10 @@ For `lnd`:
   After:
 
   ```text
+  # New flag to tell LiT to run its own lnd in integrated mode. We need to set
+  # this because "remote" is the new default value if we don't specify anything.
+  lnd-mode=integrated
+
   # Application Options
   lnd.alias=merchant