Add a SECURITY.md file documenting how to report vulnerabilities

[Sulex45]

There is currently no guidance for contributors or users who discover a security vulnerability in StellarKit API. Add a SECURITY.md file at the repo root that explains the responsible disclosure process — how to report a vulnerability privately, what information to include, and what response timeline to expect. Link it from the README.

Acceptance Criteria: SECURITY.md exists at the repo root with a clear reporting process. Linked from the README under a "Security" section.

[fairbid01]

@fairbid01 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

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[olufunsoolutayo]

@olufunsoolutayo has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello Maintainer. I would love to resolve this issue. Please assign to me. Thank you

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[shakurJJ]

@shakurJJ has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I would like to work on this issue

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[drips-wave]

Congratulations, @shakurJJ! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @shakurJJ: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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