The README.md was providing confusing guidance for Synology NAS users by linking to a generic Synology guide that pointed to DSM 7 native package instructions, which doesn't help DSM 6 users who must use Docker. Additionally, the documentation was missing important environment variable information for DSM 6 setups.

Changes Made

Clear DSM Version Distinction: Updated the Step-By-Step Guides section to clearly separate installation methods:

• DSM 6: Must use Docker with link to specific DSM 6 Docker wiki page
• DSM 7: Can use native package through Package Center

DSM_HOSTNAME Environment Variable: Added documentation for the DSM_HOSTNAME environment variable, which is required for proper mDNS configuration on Synology DSM 6 systems. This variable is supported in the container's setup script but was not documented.

Synology-Specific Docker Compose Example: Added a complete docker-compose configuration specifically for DSM 6 users that includes the DSM_HOSTNAME environment variable:

version: '2'
services:
  homebridge:
    image: homebridge/homebridge:latest
    restart: always
    network_mode: host
    environment:
      - DSM_HOSTNAME=your-synology-hostname
    volumes:
      - ./volumes/homebridge:/homebridge
    # ... health checks and logging

Clear Usage Guidance: Added a prominent note explaining that DSM 6 users must use this Docker image while DSM 7 users should prefer the native package for better integration.

These changes address the confusion around Synology installation methods and provide current, accurate information about required environment variables (confirming that S6_OVERLAY_VERSION is still in use at version 3.2.0.2, and documenting DSM_HOSTNAME support).

Fixes #660.

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