Improve Docker Compose documentation and example (#95)

- Clean up the example `docker-compose.yml` file by removing unnecessary
configuration/comments and add missing volume declaration
- Update the project README.md with instructions on how to use the
project with Docker Compose

docker-compose.yml

@@ -3,16 +3,12 @@ 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:/root
 volumes:
   protonmail:
     name: protonmail