Add TPU README
# What does this PR do?

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Adds intructions on how to access a TPU. 

@patil-suraj - can you check whether it works as expected? Can you follow the steps of the PR? I've sent you two mails to @huggingface.co

@avital - could you also review? :-)


LGTM!

One thing -- In the email you sent to others, or in this README, or both we should say that "please only SSH into machines where you're collaborating on a project. If there are any concerns about access, we may look at the audit logs to see who accessed which TPUs" or something like that.