Write `notifications/README.md` with integration guide

[Nursca]

Description
There is no documentation for the notifications service beyond a brief entry in docs/notifications.md. Integrators need a complete guide covering setup, webhook registration, event types, and HMAC signature verification.

Requirements and context

• Service overview and architecture
• Environment variable reference
• Webhook registration walkthrough with curl examples
• Full event type reference with example payloads
• HMAC signature verification code examples (TypeScript, Python, Go)
• Email subscription setup guide

Suggested execution

git checkout -b docs/notifications-readme

• Create notifications/README.md
• Write HMAC verification examples in multiple languages
• Link from org root README.md

Example commit message
docs(notifications): write comprehensive notifications service README

[Abba073]

@Abba073 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, i would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Abba073 to this issue.

[distributed-nerd]

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

can i work on this

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

[grantfox-oss]

🦊 GrantFox — @distributed-nerd has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #273)
2. Your PR will be reviewed by the Invoice-Liquidity-Network maintainers

Good luck! Track your progress on GrantFox.

[drips-wave]

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

🧑‍💻 @distributed-nerd: 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.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@distributed-nerd's PR #341 was approved and merged by @Levi-Ojukwu.

🏆 @distributed-nerd: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (401 total points). Track your full progress on GrantFox.

👏 Great work, @distributed-nerd! Keep contributing to Invoice-Liquidity-Network.