Improve requester-pays documentation

[zaneselvans]

Working with requester-pays data requires a little bit of cloud setup, which may be a challenge for some users. Update instructions based on suggestions from @cmgosnell in #23

• Create a separate page in the docs explaining how to set up requester pays.
• Grab some screenshots from the GCP console for setting up the project, setting IAM permissions.
• Make it clear which options to choose when doing gcloud init
  • Re-initialize this configuration with new settings
  • Pick the cloud project that you just created in GCP
• Update README to point at requester pays documentation for setup.