expand docs for proxy nginx config

docker/shared/root/docker/templates/docker/shared/proxy/conf.d/docker-pixelfed.conf

@@ -3,5 +3,13 @@
 ###########################################################
 # This file is generated by the Pixelfed Docker setup, and
 # will be rewritten on every container start
+#
+# You can put any .conf file in this directory and it will be loaded
+# by nginx on startup.
+#
+# Run [docker compose exec proxy bash -c 'nginx -t && nginx -s reload']
+# to test your config and reload the proxy
+#
+# See: https://github.com/nginx-proxy/nginx-proxy/blob/main/docs/README.md#custom-nginx-configuration
 
 client_max_body_size {{ getenv "POST_MAX_SIZE" "61M" }};