docs: mention MegaLinter in the README

[nvuillam]

Description:

Hi! I maintain MegaLinter, an open-source linters aggregator for CI that ships TruffleHog as one of its built-in security scanners. This PR just adds a short note about it in the README, next to the other CI integrations (GitHub Action, GitLab CI), so TruffleHog users know they can also get it through MegaLinter: https://megalinter.io/latest/descriptors/repository_trufflehog/

Happy to tweak wording or placement if you'd prefer it elsewhere.

Checklist:

• Tests passing (make test-community)? — docs-only change
• Lint passing (make lint)? — docs-only change

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Note

Low Risk
Documentation-only README change with no runtime or security impact.

Overview
Adds a MegaLinter subsection to the README’s CI integrations area (after GitLab CI, before Pre-commit Hook).

It notes that TruffleHog ships inside MegaLinter so teams can run it alongside other linters in one CI job, and links to MegaLinter’s TruffleHog descriptor docs.

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