[css-backgrounds-4][editorial] Removed the "Not Ready For Implementation" status

Author: SebastianZ

css-backgrounds-4/Overview.bs

@@ -15,7 +15,6 @@ Ignored Vars: width of background positioning area, width of background image, h
 Ignored Terms: total width
 WPT Path Prefix: css/css-backgrounds/
 WPT Display: closed
-Warning: Not Ready
 </pre>
 
 <pre class="link-defaults">
@@ -1637,6 +1636,16 @@ Sizing Images: the 'background-size' property</h3>
 <h3 id='background-layers'>
 Background Image Layers: the 'background-tbd' shorthand property</h3>
 
+	<details>
+		<summary>Not Ready For Implementation</summary>
+		<p>
+		This section is not yet ready for implementation.
+		It exists in this repository to record the ideas and promote discussion.
+		<p>
+		Before attempting to implement anything of this section,
+		please contact the CSSWG at [email protected].
+	</details>
+
 	<pre class="propdef">
 		Name: background-tbd
 		Value: <<bg-layer>>#