Add README with 6RD setup context and provider adaptation guidance

[Copilot]

The repository had only a one-line README with no context on supported hardware, firmware, ISP, or how to adapt the scripts.

Changes

• Overview — documents target hardware (Unifi Cloud Gateway), tested firmware (v5.0.16), and primary ISP (CenturyLink)
• Usage — outlines the steps to deploy the script on a gateway
• Adapting for Other Providers — calls out the specific variables in centurylink-6rd-setup.sh to update for non-CenturyLink 6RD providers (relay address, prefix, prefix length, IPv4 mask length)
• Requirements & License — firmware version requirement and link to LICENSE

Original prompt

Draft a readme that explains that this is a script for Unifi Cloud Gateway and was tested on firmware v5.0.16. The script is for 6RD and developed for Century Link as a provider, however should be easily adapted to others by updating the variables in the centurylink-6rd-setup.sh file.

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