Add radixtree link to README

[gammazero]

Added link to adaptive radix tree implementation.

Required links

Provide the links below. Our CI will automatically validate them.

• Forge link (github.com, gitlab.com, etc): https://github.com/gammazero/radixtree
• pkg.go.dev: https://pkg.go.dev/github.com/gammazero/radixtree
• goreportcard.com: https://goreportcard.com/report/github.com/gammazero/radixtree
• Coverage service link (codecov, coveralls, etc.): https://app.codecov.io/gh/gammazero/radixtree

Pre-submission checklist

• I have read the Contribution Guidelines
• I have read the Quality Standards

Repository requirements

These are validated automatically by CI:

• The repo has a go.mod file and at least one SemVer release (vX.Y.Z).
• The repo has an open source license.
• The repo documentation has a pkg.go.dev link.
• The repo documentation has a goreportcard link (grade A- or better).
• The repo documentation has a coverage service link.

These are recommended and reported as warnings:

• The repo has a continuous integration process (GitHub Actions, etc.).
• CI runs tests that must pass before merging.

Pull Request content

These are validated automatically by CI:

• This PR adds/removes/changes only one package.
• The package has been added in alphabetical order.
• The link text is the exact project name.
• The description is clear, concise, non-promotional, and ends with a period.
• The link in README.md matches the forge link above.

Category quality

Note: new categories require a minimum of 3 packages.

Packages added a long time ago might not meet the current guidelines anymore. It would be very helpful if you could check 3-5 packages above and below your submission to ensure they still meet the Quality Standards.

Please delete one of the following lines:

• The packages around my addition still meet the Quality Standards.
• I removed the following packages around my addition: (please give a short reason for each removal)

Thanks for your PR, you're awesome! 😎

[github-actions]

Automated Quality Checks

Required checks

✅ Repo: accessible, has go.mod and SemVer release
✅ pkg.go.dev: OK
✅ Go Report Card: OK (grade unknown)

Additional checks

✅ License: MIT
✅ Maturity: repo has 5+ months of history
✅ CI/CD: GitHub Actions workflows detected
✅ README: present
⚠️ Coverage: missing from PR body

How to fix: Add a coverage service link to your PR description:

Coverage: https://app.codecov.io/gh/your-org/your-project

Popular options: [Codecov](https://codecov.io), [Coveralls](https://coveralls.io). Integrate one with your CI to track coverage automatically.


_These checks are automated and do not replace maintainer review. See [CONTRIBUTING.md](https://github.com/avelino/awesome-go/blob/main/CONTRIBUTING.md) for full guidelines._

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## PR Diff Validation

### Content checks

✅ **Files changed**: only README.md
✅ **Single item**: one package added
✅ **Link consistency**: README link matches forge link in PR body
✅ **Link text**: matches repository name
✅ **Description**: ends with punctuation
✅ **Description tone**: no promotional language detected
✅ **Category size**: Text Analysis has 10 items


_Automated diff validation — does not replace maintainer review._
<!-- Sticky Pull Request Commentpr-quality-check -->