Update README

Change-Id: Icd8dfbc9fd7c00d689951021c1f6d9834b5eb804

README.md

-Usage example:
+Usage example on arm64 host:
 
-apt update; apt install --no-install-recommends fai-server fai-setup-storage qemu-utils procps mtools
+sudo modprobe loop
+docker run --privileged  -v $(pwd):/work --rm -it linaro/arm64-fai  /bin/bash
+cd /work
 git clone http://git.linaro.org/ci/fai.git
-fai-diskimage -v --cspace $PATH/fai  --hostname linaro-test -S 3G --class SID,DEBIAN,LINARO,QCOM,DB410C,RAW linaro.raw 2>&1|tee fai.log
-img2simg linaro.raw linaro.img
+fai-diskimage -v --cspace $(pwd)/fai  --hostname linaro-test -S 3G --class BUSTER,DEBIAN,DEVELOPER,LINARO,QCOM,DB410C,RAW linaro.raw 2>&1|tee fai.log
 
 Gotchas:
+- loop devices need to exist before starting docker
 - order of classes matter - latter classes override variables from earlier ones
 - missing packages in packages from package\_lists and errors while installing packages will not error the build out
   - grep the log for warnings and erros before proceeding