add README

SVN-Revision: 23925

target/linux/realview/README

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+This ARM Ltd. Realview target is intended to be used with the Qemu emulator.
+It can be used to prototype an OpenWrt firmware targetting ARM-based hardware.
+One could also use it to troubleshoot ARM applications without access to real
+hardware.
+
+To use the images built by OpenWrt with qemu, use the following commands:
+
+qemu-system-arm -M realview-eb-mpcore \
+-kernel bin/realview/openwrt-realview-vmlinux.elf \
+-nographic -m 128
+
+and enjoy the system booting.