Describe JSON serialization of metadata in docs

Sources/Logging OSLog/Docs.docc/index.md

@@ -66,6 +66,13 @@ func init() {
 		"main": Logger(label: "com.example.yourapp.Main"),
 		"mail": Logger(label: "com.example.yourapp.Mail System"),
 	]
+
+	// And using loggers
+	logger["main"]!.debug("Starting build",
+		metadata: [
+			"input.output_directory": "\(output)",
+			"input.data": "\(data ?? "-")"
+	])
 }
 ```
 
@@ -78,6 +85,9 @@ library maps a reverse domain style label to the subsystem and category
 parameters of the unified logging system. See
 ``/LoggingOSLog/LoggingOSLog/init(label:)`` for more information.
 
+Metadata is appended as JSON to the logged message after an ASCII right
+arrow ("->").
+
 ### What this library does not supply
 This library does not implement any of the interpolation privacy features of
 the unified logging system, i.e. there is no way to redact or align a meta