[WIP] Developer Guide: Add instruction for gcloud python dep.

[wbuchwalter]

Some dependencies such as kubernetes and google-cloud need to be installed via pip before we can run the release script documented in the developer guide.

Alternatively we could create a requirements.txt file.

[k8s-ci-robot]

Hi @wbuchwalter. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[wbuchwalter]

Closed in favor of #180 which I didn't see :)