Update CHANGELOG.md (rust-windowing#1186)

Author: goddessfreya

CHANGELOG.md

@@ -6,6 +6,14 @@
 
 - Update winit dependency to 0.20.0-alpha1. See [winit's CHANGELOG](https://github.com/rust-windowing/winit/blob/master/CHANGELOG.md#0200-alpha-1) for more info.
 
+# Version 0.21.0 (2019-04-20)
+
+ - Bumped dependencies, fixed docs.
+
+# Version 0.21.0-rc3 (2019-04-13)
+
+ - Bumped dependencies.
+
 # Version 0.21.0-rc2 (2019-04-08)
 
  - **Breaking**: Removed `DisplayLost` variant to `ContextError`.