Basilisk II: Update README to cover bridging tun interfaces.

Now that issue #154 has been fixed, when Basilisk II is configured to
use "tun" for networking, the generated tunN interface can be bridged,
enabling AppleTalk frames to be forwarded without the Linux host
needing to be able to route AppleTalk.

This commit updates the README to discuss this possibility and briefly
outline how it can be achieved.  A number of spelling corrections and
other minor clarifications are made in the same area.

BasiliskII/README

@@ -442,25 +442,43 @@ ether <ethernet card description>
          instead of sending packets via physical media writes them to
          the user space program.
 
-         Prerequesties:
+         Prerequisites:
          - Make sure the "tun" kernel module is loaded
-           # modprobe tun
-         - Make sure IP Fordwarding is enabled on your system
-           # echo 1 >/proc/sys/net/ipv4/ip_forward
 
-         A virtual network configuration script is required and the
+             # modprobe tun
+
+         - If you wish to route IP packets from Basilisk II, make sure
+           IP Forwarding is enabled on your system (not required for
+           bridging)
+
+             # echo 1 >/proc/sys/net/ipv4/ip_forward
+
+         A virtual network configuration script is required in order
+         to configure the tunN interface after it is created, and the
          default is /usr/local/BasiliskII/tunconfig unless you specify
          a different file with the "etherconfig" item.
 
-         This script requires you that "sudo" is properly configured
+         The default "tunconfig" script configures the tunN interface
+         for IP NAT.  It requires that "sudo" is properly configured
          so that "/sbin/ifconfig" and "/sbin/iptables" can be executed
          as root. Otherwise, you can still write a helper script which
-         invokes your favorite program to enhance a user priviledges.
+         invokes your favorite program to elevate user privileges.
          e.g. in a KDE environment, kdesu can be used as follows:
 
            #!/bin/sh
            exec /usr/bin/kdesu -c /path/to/tunconfig $1 $2
 
+         As an alternative to configuring IP on the tunN interface,
+         you may attach it to a bridge, which will enable AppleTalk
+         frames to be forwarded without Linux needing to route them.
+         No tunconfig-like script is provided to configure bridging,
+         but you may simply configure "etherconfig /bin/true" to skip
+         the automatic configuration and then configure bridging
+         manually once Basilisk II has started, e.g.:
+
+           # ifconfig tun0 up
+           # brctl addif bridge1 tun0
+
       4. Access the network through the user mode network stack.
          (the code and this documentation come from QEMU)