add README

README.md

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+Holesail-docker proxies traffic from another container.
+
+The easiest way to do this is to create a network in Docker:
+```
+docker network create proxy
+```
+
+You can then use the contaier name and port of the container you want to proxy, as exampled below.
+
+```
+---
+services:
+  holesail:
+    container_name: holesail
+    restart: unless-stopped
+    build: .
+    environment:
+      MODE: server      # defaults to client
+      PORT: 25565
+      HOST: minecraft   # defaults to 0.0.0.0
+      CONNECTOR: very-super-secret # leave this blank to generate a random secret.
+    networks:
+      - proxy
+
+  mc:
+    image: itzg/minecraft-server
+    container_name: minecraft
+    tty: true
+    stdin_open: true
+    restart: unless-stopped
+    ports:
+      - "25565:25565"
+    environment:
+      EULA: "TRUE"
+    volumes:
+      # attach the relative directory 'data' to the container's /data path
+      - ./data:/data
+    depends_on:
+      - holesail
+    networks:
+      - proxy
+
+networks:
+  proxy:
+    external: true 
+```