doc: add note for the directory of UOS image

doc/tutorials/using_ubuntu_as_sos.rst

@@ -63,8 +63,6 @@ Install development tools for ARCN development:
      sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-7 60 \
                      --slave /usr/bin/g++ g++ /usr/bin/g++-7
      
-     
-     
 Install ACRN
 ************
 
@@ -313,6 +311,10 @@ For the User OS, we are using the same `Clear Linux`_ release version as the Ser
      -s 3,virtio-blk,/root/clear-26440-kvm.img
      -k /lib/modules/kernel/default-iot-lts2018
         
+.. note::
+  The image of UOS can be stored in other directories instead of "/root", 
+  and please remember to modify it in "launch_uos.sh" too.
+
 Start the User OS (UOS)
 ***********************