Add decant-core resource link to README

[deadrat-in]

What does this PR add or change?

Added a new resource link for decant-core to the README.

Checklist

• [x ] The resource is real, actively maintained, and on-topic for this list.
• [ x] The link uses the canonical URL (official site or GitHub repo).
• [ x] The entry follows the format: - [Name](url) - One concise sentence ending with a period.
• [ x] It is placed in the most relevant section and is not a duplicate.
• [ x] No broken links, and the description avoids marketing fluff.