Add border radius release notes

[chompaa]

Fixes #1246.

I also have some questions:

• Are the doc links correct?
• Is the example too bloated?
• Are these notes too brief?

[alice-i-cecile]

Are the doc links correct?

Can you swap to https://dev-docs.bevyengine.org/bevy/index.html instead for now? That way they'll work before the release, and we can do a final find-and-replace during editing.

Is the example too bloated?

No, this is solid.

Are these notes too brief?

I like the brevity. This is a simple well-understood feature, even if the impact is high.

[chompaa]

Can you swap to https://dev-docs.bevyengine.org/bevy/index.html instead for now? That way they'll work before the release, and we can do a final find-and-replace during editing.

This makes much more sense, thanks! I've updated the links :)