Add docker-compose file for convenience

This is functionally identical to the docker run command in the readme.

docker-compose.yml

@@ -0,0 +1,13 @@
+version: '3'
+services:
+  zerotier:
+    image: zyclonite/zerotier
+    container_name: zerotier-one
+    devices:
+      - /dev/net/tun
+    network_mode: host
+    volumes:
+      - '/var/lib/zerotier-one:/var/lib/zerotier-one'
+    cap_add:
+      - NET_ADMIN
+      - SYS_ADMIN