Add a docker compose file (#70)

It's quite the norm to include a docker-compose file, generally in the
README or the root for people to copy and modify. For example as
https://github.com/wfg/docker-openvpn-client has done so.

If there are [Environmental
variables](https://github.com/wfg/docker-openvpn-client#environment-variables),
they should also be documented - in this case there isn't.

docker-compose.yml

@@ -0,0 +1,18 @@
+version: '2.1'
+
+services:
+  protonmail-bridge:
+    image: shenxn/protonmail-bridge
+    # build:
+    #  context: ./build
+    #  dockerfile: Dockerfile
+    container_name: pm_bridge
+    ports:
+      - 1025:25/tcp
+      - 1143:143/tcp
+    restart: unless-stopped
+    stdin_open: true 
+    tty: true
+volumes:
+  protonmail:
+    name: protonmail