docs: added example of auto-cf custom manifest (#2582)

examples/auto-cf/modpack-manifest/docker-compose.yml

@@ -0,0 +1,19 @@
+services:
+  mc:
+    image: itzg/minecraft-server
+    environment:
+      EULA: true
+      MOD_PLATFORM: AUTO_CURSEFORGE
+      # allocate from https://console.curseforge.com/ and set in .env file
+      CF_API_KEY: ${CF_API_KEY}
+      CF_MODPACK_MANIFEST: /manifests/manifest.json
+      CF_SLUG: "custom"
+      MEMORY: 2G
+    volumes:
+      - mc-data:/data
+      - ./manifests:/manifests:ro
+    ports:
+      - "25565:25565"
+
+volumes:
+  mc-data: {}