Add high-level documentation

https://nostr-ruby.com

docs/events/text-note.md

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+# Text Note
+
+In the `Text Note` event, the `content` is set to the plaintext content of a note (anything the user wants to say).
+Content that must be parsed, such as Markdown and HTML, should not be used.
+
+## Sending a text note event
+
+```ruby
+text_note_event = user.create_event(
+  kind: Nostr::EventKind::TEXT_NOTE,
+  content: 'Your feedback is appreciated, now pay $8'
+)
+
+client.publish(text_note_event)
+```