Sponsorship Model Explainer in README

[Sulex45]

Description:
Add a section explaining Stellar's account sponsorship model — what it is, why it exists, and how developers can use the sponsorship endpoint to understand sponsored accounts.
Acceptance Criteria:

New section titled "Understanding Account Sponsorship"
Explains what sponsorship means in Stellar context
Explains sponsored reserves and who pays the base reserve
References the /account/:id/sponsorship endpoint
No code changes

[Achievers-sketch]

@Achievers-sketch has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

I’d like to work on this issue because clear documentation helps developers better understand Stellar concepts and APIs. I have experience writing technical documentation, simplifying blockchain concepts, and creating developer-friendly guides that improve onboarding and usability.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Achievers-sketch to this issue.

[drips-wave]

Congratulations, @Achievers-sketch! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @Achievers-sketch: 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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