Add a note about nginx accessing the public directory

[Hex052]

I had this issue and it was not incredibly clear as I followed the instructions exactly. However it appears either DigitalOcean or Ubuntu changed the default permission for /home/username folders to not be readable to users other than themselves.

[vercel[bot]]

@Hex052 is attempting to deploy a commit to the Mastodon Team on Vercel.

A member of the Team first needs to authorize it.

[aaaaalbert]

See also #1128 (same problem, plus link to suggested solution).

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.

[github-actions[bot]]

This pull request has resolved merge conflicts and is ready for review.