[archiso] Update README (know issues)

Signed-off-by: Gerardo Exequiel Pozzi <vmlinuz386@yahoo.com.ar>

README

@@ -435,12 +435,14 @@ When make your custom boot-pendrive, you need to copy /arch directory to it.
 
 *** Know issues
 
-** (1) On shutdown there are two steps that [FAIL]:
-   "Unmounting Swap-backed Filesystems" and "Unmounting Non-API Filesystems",
-    These filesystem are in use at this stage by archiso, but...
+** (1) On shutdown lots of messages from systemd like:
+
+    "Could not unmount /run/archiso/<ABC>: Device or resource busy"
+    "Could not delete loopback /dev/loop<N>: Device or resource busy"
     This is not a real issue since, all mounted filesystem, loopback devices
     and device mapper devices made by archiso will be "free" on "shutdown tmpfs"
     (A.K.A deinitramfs), build at initramfs by [archiso_shutdown] initcpio hook.
+    Proper shutdown is mostly important when persistent is used.
 
 ** (2) syslinux 4.05 bug with relative directories on FAT: