add gpg key dependency to docs

[amy]

Signed-off-by: Amy Chen [email protected]

What this PR does / why we need it:
Updates the documentation to instruct users to have a gpg key associated with their github profiles to enable the release script to work.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Related to: #795

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[k8s-ci-robot]

Hi @amy. Thanks for your PR.

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[chuckha]

/ok-to-test

[chuckha]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: amy, chuckha

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