Add border radius release notes (#1353)

Author: chompaa

release-content/0.14/release-notes/12500_Add_border_radius_to_UI_nodes_adopted.md

@@ -1 +1,29 @@
-TODO
+Border radius for UI nodes has been a long-requested feature for Bevy. Now it's supported!
+
+To apply border radius to a UI node, there is a new component [`BorderRadius`](https://dev-docs.bevyengine.org/bevy/prelude/struct.BorderRadius.html). The [`NodeBundle`](https://dev-docs.bevyengine.org/bevy/prelude/struct.NodeBundle.html) and [`ButtonBundle`](https://dev-docs.bevyengine.org/bevy/prelude/struct.ButtonBundle.html) bundles have a new field in place for this called `border_radius`. For example:
+
+```rs
+commands.spawn(NodeBundle {
+    style: Style {
+        width: Val::Px(50.0),
+        height: Val::Px(50.0),
+        // We need a border to round a border, after all!
+        border: UiRect::all(Val::Px(5.0)),
+        ..default()
+    },
+    border_color: BorderColor(Color::BLACK),
+    // Apply the radius to all corners. 
+    // Optionally, you could use `BorderRadius::all`.
+    border_radius: BorderRadius {
+        top_left: Val::Px(50.0),
+        top_right: Val::Px(50.0),
+        bottom_right: Val::Px(50.0),
+        bottom_left: Val::Px(50.0),
+    },
+    ..default()
+});
+```
+
+There's a [new example](https://github.com/bevyengine/bevy/blob/main/examples/ui/rounded_borders.rs) showcasing this new API, a screenshot of which can be seen below:
+
+![`rounded_borders` example](./rounded_borders.png)