Updated the instruction of running UEFI shell commands and script

Added more information about running UEFI shell commands script debug_dump.nsh
Added a folder for the logs collected by manually running debug_dump.nsh
Note that after new ACS image gets released, some manual-run commands like dh, bcfg, and so on in the "./fw/uefi-shell-cmds-logs/" section will be removed .

Signed-off-by: Sunny Wang <sunny.wang@arm.com>