readm.exe is used to enable local built in administrator? if yes what are settings need to configure in windows 7 image or system image manager

I wouldn't use stuff like that when there are many other methods to achieve something similar. For some examples see: https://social.technet.microsoft.com/Forums/en-US/2dedd3a4-ed6c-4b61-9937-f22d963c4bcb/enable-builtin-administrator-account?forum=w7itproinstall

Also, when you're using ConfigMgr you can simply enable it directly during the task sequence.

you are correct , but once machine is joined to domain built in administrator account will be disabled so how to enable  built in administrator account.

There's a reason why built in administrator is disabled. Why don't you just add local user and add it to local admins group during the deployment? This can be done either via unattended.xml or custom script. You could also use restricted groups to add one of your AD groups as member of local administrators group through group policies.