Remove docs/README.knownissues. Replaced by issue #83.

docs/README.knownissues

-*** Know issues
-
-** (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.
-
-