Merge pull request #538 from yasminvalim/release-notes0.21.0

docs/release-notes.md: update for release Butane 0.21.0

Author: travier

docs/release-notes.md

@@ -4,7 +4,13 @@ nav_order: 9
 
 # Release notes
 
-## Upcoming Butane 0.21.0 (unreleased)
+## Upcoming Butane 0.22.0 (unreleased)
+
+
+## Butane 0.21.0 (2024-06-06)
+
+Starting with this release, Butane binaries are signed with the [Fedora 40
+key](https://getfedora.org/security/).
 
 
 ### Breaking changes