Docs: Added guide for clustering

[NIREKI]

Hey there, I've added a guide for clustering/ running multiple instances to the Readme.md file.

Added/Changed:

• Removed version from docker-compose.yml because it's now officially obsolete
• Added ToC to readme.md
• Added Guide for clustering to readme.md including
  • changes to the .env file
  • addition of the COMPOSE_PROJECT_NAME env variable to ensure docker compose compatibility
  • addition of clusterID according to the ark patchnotes
  • Fixed chown command

@azixus If you want to further change the .env file (like adding the COMPOSE_PROJECT_NAME from the beginning), I'm open to discuss 👍