Document CORS configuration

[webmaster128]

When using an nginx reverse proxy:

add_header Access-Control-Allow-Origin * always;
add_header Access-Control-Allow-Headers * always;

For config.toml

# A list of origins a cross-domain request can be executed from
# Default value '[]' disables cors support
# Use '["*"]' to allow any origin
cors_allowed_origins = ["*"]