Add release notes for per-side UI border colors (#21154)

# Objective

- @cart wanted a quick release note for #18682
- Fixes #21124

## Solution

- Write some brief release notes, explaining the most common
application.

Author: alice-i-cecile

release-content/release-notes/polychromatic-borders.md

@@ -0,0 +1,14 @@
+---
+title: Per-side UI border colors
+authors: ["@robtfm"]
+pull_requests: [18682]
+---
+
+TODO: add image from 18682
+
+`bevy_ui` now supports distinct border colors on each side of your UI nodes,
+controlled with the [`BorderColor`] component.
+This feature was borrowed from CSS, where it is commonly used to fake buttons with depth,
+but we're looking forward to seeing your creative designs.
+
+[`BorderColor`]: https://docs.rs/bevy/0.17.0-rc.1/bevy/prelude/struct.BorderColor.html