Add suggestions for configuring CORS on direct uploads with ActiveStorage

[loop23]

References #31523

Summary

Modified README.md to include suggestions on configuring CORS on remote (in this case, GCS) to allow direct uploads with ActiveStorage

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @kaspth (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

This repository is being automatically checked for code quality issues using Code Climate. You can see results for this analysis in the PR status below. Newly introduced issues should be fixed before a Pull Request is considered ready to review.

Please see the contribution instructions for more information.

[loop23]

Sorry I didn't realize a rails guide for ActiveStorage exists. My suggestion clearly belongs there rather than in the README.

[kinnrot]

@loop23, did you tried and put this on rails guides? I don't see any cors mentioned there, and this really helped me.

[loop23]

@kinnrot nope sorry. Too busy at the moment, glad it helped anyway