Add documentation for proxy example

.examples/proxy/docker-compose.yml

@@ -0,0 +1,16 @@
+version: '3.3'
+services:
+  frontend:
+    image: jwilder/nginx-proxy:alpine
+    restart: always
+    ports:
+      - "80:80"
+      - "443:443"
+    volumes:
+      - ./certs:/etc/nginx/certs
+      - /var/run/docker.sock:/tmp/docker.sock:ro
+    networks:
+    - 2_apps
+
+networks:
+  2_apps:

README.md

@@ -18,3 +18,17 @@ https://docs.zammad.org/en/latest/install-docker-compose.html
 ## Build Status
 
 [![Build Status](https://travis-ci.org/zammad/zammad-docker-compose.svg?branch=master)](https://travis-ci.org/zammad/zammad-docker-compose)
+
+## Using a reverse proxy
+
+In environments with more then one web applications it is necessary to use a reverse proxy to route connections to port 80 and 443 to the right application.
+To run Zammad behind a revers proxy, we provide `docker-compose.proxy-example.yml` as a starting point.
+
+1.  Copy `./.examples/proxy/docker-compose.proxy-example.yml` to your own configuration, e.g. `./docker-compose.prod.yml`  
+    `cp ./.examples/proxy/docker-compose.proxy-example.yml ./docker-compose.prod.yml`
+1.  Modify the environment variable `VIRTUAL_HOST` and the name of the external network in `./docker-compose.prod.yml` to fit your environment.
+1.  Run docker-composer commands with the default and your configuration, e.g. `docker-compose -f docker-compose.yml -f docker-compose.prod.yml up -d`  
+
+See `.examples/proxy/docker-compose.yml` for an example proxy project.
+
+Like this, you can add your `docker-compose.prod.yml` to a branch of your Git repository and stay up to date by merging changes to your branch.