docs: add instructions for using the with-nginx-proxy-external-ssl Docker compose

Signed-off-by: Egidio Caprino <egidio.caprino@gmail.com>

.examples/docker-compose/with-nginx-proxy-external-ssl/mariadb/fpm/README.md

@@ -0,0 +1,16 @@
+You need to have an SSL certificate for your domain. You could generate it with [Certbot](https://certbot.eff.org/).
+
+Fill in the `db.env` and `ssl.env` files and the `MYSQL_ROOT_PASSWORD` environment variable in `docker-compose.yml` file. The `CERT_PATH` needs to contain the
+
+- `.crt` file
+- `.key` file
+
+If you generated your certificate with Certbot you can create symbolic links:
+
+```shell
+cd /path/to/certs
+ln -s /path/to/certs/fullchain1.pem mydomain.crt
+ln -s /path/to/certs/privkey1.pem mydomain.key
+```
+
+Replace `mydomain` with the domain covered by your certificate and used to access your Nextcloud instance.