Add Gitpod Troubleshooting Steps to the Readme

[jtfairbank]

See this PR from our backend project, seems useful to have here as well: distributeaid/aggregated-public-information#116

We may also want to include instructions to "pin" workspaces on gitpod so that they aren't auto-deleted after 2 weeks of inactivity. Ideally folks don't leave unpushed work lying around for 2 weeks, but a good step to take just in case. See the Gitpod documentation on workspace pinning here, and good to add screenshots, etc like the backend PR did for the Github permissions.