Update README for case sensitivity bug

[gatorcse]

Many guides around the internet, including in this repositories own issue history, suggest using gcloud config get-value account to get the email address. The --user option is case sensitive with respect to the email address provided. This will cause the sc install to fail with a verbose permissions error that makes it look like the clusterrolebinding was not created. This message will give guidance that users should check the casing of the email address provided.

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