Add high-level documentation

https://nostr-ruby.com

docs/events/set-metadata.md

@@ -0,0 +1,20 @@
+# Set Metadata
+
+In the `Metadata` event, the `content` is set to a stringified JSON object
+`{name: <username>, about: <string>, picture: <url, string>}` describing the [user](../core/user) who created the event. A relay may
+delete older events once it gets a new one for the same pubkey.
+
+## Setting the user's metadata
+
+```ruby
+metadata_event = user.create_event(
+  kind: Nostr::EventKind::SET_METADATA,
+  content: {
+    name: 'Wilson Silva',
+    about: 'Used to make hydrochloric acid bombs in high school.',
+    picture: 'https://thispersondoesnotexist.com/'
+  }
+)
+
+client.publish(metadata_event)
+```