docs: add custom events howto

[tcheeric]

Summary

• document how to manually build and publish non-standard Nostr events

Testing

• mvn -q verify (fails: Could not find a valid Docker environment)

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https://chatgpt.com/codex/tasks/task_b_68a48d9bbdf08331a313fcb945fe393b

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[Copilot]

Pull Request Overview

This PR adds documentation for creating and publishing custom Nostr events using the nostr-java library. The guide explains how to work with non-standard event kinds while adhering to the Nostr protocol specifications.

• Adds a comprehensive how-to guide for custom Nostr events
• Provides a practical Java code example demonstrating event creation, signing, and publishing
• Documents the required event fields and references relevant NIPs (Nostr Improvement Proposals)

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[Copilot]

The example uses a placeholder relay URL 'wss://relay.example.com' which is not a real relay. Consider using a well-known public relay like 'wss://relay.damus.io' or add a note that users should replace this with an actual relay URL.

Suggested change

	try (StandardWebSocketClient client = new StandardWebSocketClient("wss://relay.example.com")) {
	try (StandardWebSocketClient client = new StandardWebSocketClient("wss://relay.damus.io")) {