readme.md: update schema

readme.md

@@ -5,6 +5,11 @@ This repo contains backing information for Glowers Radio.
 The `songs.json` format **is only intended for use for Glowers Radio**, and is effectively a living standard as a result. **Live data may change drastically without warning**
 
 ```yaml
+required: [ version, songs]
+properties:
+  version: { type: integer }
+  songs:
+    items:
       required: [ id, sources ]
       properties:
         id:
@@ -24,6 +29,6 @@ properties:
             properties:
               type: { type: string, format: mimetype }
               url: { type: string, format: uri }
-        thumbnail_type: { type: string }
+              thumbnail_type: { type: number }
               thumbnail_url: { format: uri }
 ```