Added SETUP.md

SETUP.md

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+# NGINX - External Proxy
+
+For setups behind an external reverse proxy (heavily recommended), using a local CA and self-signed certificates is not required, but also recommended.
+
+After installing `mkcert` through your system package manager:
+
+```bash
+mkcert -install
+mkcert abc.domain.xyz, bcd.domain.xyz
+mv domain.xyz+X-key.pem nginx/certs/key.pem
+mv domain.xyz.pem nginx/certs/certificate.pem
+```
+
+NGINX will find these certificates automatically and use them when proxying the application. Unless you know what you are doing, editing files under `nginx/configs/` is not recommended.
+
+If preferred, NGINX can be used as a reverse proxy, however an additional `certbot/certbot:latest` container would be required unless other valid SSL certificates are provided under `nginx/certs`.