Update busybox documentation

Convert README.busybox to markdown and add the note on the sysctl applet
incompatibility.

BUSYBOX.md

@@ -0,0 +1,32 @@
+# Using Busybox as your Default Shell with OpenRC
+
+If you have/bin/sh linked to busybox, you need to be aware of several
+incompatibilities between busybox's applets and the standalone
+counterparts. Since it is possible to configure busybox to not include
+these applets or to prefer the standalone counterparts, OpenRC does not
+attempt to support the busybox applets.
+
+For now, it is recommended that you disable the following busybox
+configuration settings for best results with OpenRC.
+
+CONFIG_START_STOP_DAEMON -- The start-stop-daemon applet is not compatible with
+start-stop-daemon in OpenRC.
+
+CONFIG_MOUNT -- The mount applet does not support the -O [no]_netdev options to
+skip over or include network file systems when the -a option is present.
+
+CONFIG_UMOUNT -- The umount applet does not support the -O option along with -a.
+
+CONFIG_SWAPONOFF -- The swapon applet does not support the -e option
+or recognize the nofail option in fstab.
+
+CONFIG_SETFONT -- The setfont applet does not support the -u option from kbd.
+
+CONFIG_IP -- The ip applet  doesn't support the "scope" modifier for
+"ip route add" and "ip address add".
+
+CONFIG_BB_SYSCTL -- The sysctl applet does not support the --system command
+line switch.
+
+There is work to get most of these supported by busybox, so this file
+will be updated as things change.