Improve documentation with docker compose example.

I think it would be helpful to have a docker compose example for users. It may be simpler than the docker run example. If you think a docker compose suggestion would be helpful, I provided some recommended documentation for the README.md below:

Deploy SMP/XFTP servers on Linux

Docker Compose

Create a directory of your choice (e.g. ./simplexchat) to hold the compose.yml file.

mkdir ./simplexchat
cd ./simplexchat

Create a compose.yml file with the following contents. Change 1) your_ip_or_domain in both containers 2) "PASS=password" This password cannot contain special characters and 3) maximum_storage to a value such as 2GB.

---
name: simplex
services:
  simplex.smp:
    container_name: simplex.smp
    environment:
      - ADDR=your_ip_or_domain
      - PASS=password
    ports:
      - 5223:5223
    volumes:
      - ./simplex/smp/config:/etc/opt/simplex:z
      - ./simplex/smp/logs:/var/opt/simplex:z
    image: simplexchat/smp-server:latest
    restart: unless-stopped
  simplex.xftp:
    container_name: simplex.xftp
    environment:
      - ADDR=your_ip_or_domain
      - QUOTA=maximum_storage
    ports:
      - 443:443
    volumes:
      - ./simplex/xftp/config:/etc/opt/simplex-xftp:z
      - ./simplex/xftp/logs:/var/opt/simplex-xftp:z
      - ./simplex/xftp/files:/srv/xftp:z
    image: simplexchat/xftp-server:latest
    restart: unless-stopped

Then while in the same folder as the compose.yml run:

docker compose up

Once the container is running, you can find the smp and xftp server address: by viewing the docker logs.

docker logs simplex.smp

docker logs simplex.xftp

Review README.md's Linux deployment section and compare the proposed Docker Compose example with the existing docker run instructions. Add the compose.yml setup, configuration notes, startup command, and log commands shown in the issue; done means users can follow the README to run both servers and find their addresses.