Update README with instructions for using Codespaces + note about preview from fork

[rivaquiroga]

Gitpod requires a credit card for new users (it’s a refundable one dollar, but you still need a credit card). I used Codespaces to build the site for the eLife post. I’ll update the README when we’re back in 2026 to include instructions for using Codespaces.

[rivaquiroga]

Also for the README: explain that the preview doesn't work from forks