Update changelog

Author: sjakobi

CHANGES.md

@@ -1,3 +1,36 @@
+## [0.2.21] - December 2025
+
+* API enhancements:
+  * [Add `HashMap.lookupKey` and `HashSet.lookupElement`](https://github.com/haskell-unordered-containers/unordered-containers/pull/554)
+  * [Add `differenceWithKey`](https://github.com/haskell-unordered-containers/unordered-containers/pull/542)
+  * [Add `disjoint`](https://github.com/haskell-unordered-containers/unordered-containers/pull/559)
+
+* Performance improvements:
+  * [`HashSet.fromList`: Use `unsafeInsert`](https://github.com/haskell-unordered-containers/unordered-containers/pull/515)
+  * [Use tree-diffing for `difference`](https://github.com/haskell-unordered-containers/unordered-containers/pull/535)
+  * [Remove some unnecessary forcing of HashMaps](https://github.com/haskell-unordered-containers/unordered-containers/pull/545)
+  * [Optimize indexing in arrays of length 2](https://github.com/haskell-unordered-containers/unordered-containers/pull/528)
+  * [Remove the `Array.index` function](https://github.com/haskell-unordered-containers/unordered-containers/pull/539)
+  * [`hashWithSalt`: Ensure that the salt `Int` is unboxed](https://github.com/haskell-unordered-containers/unordered-containers/pull/569)
+
+* Documentation changes:
+  * [Turn some comments into docstrings](https://github.com/haskell-unordered-containers/unordered-containers/pull/516)
+  * [Reword disclaimer regarding hash collision attacks](https://github.com/haskell-unordered-containers/unordered-containers/pull/557)
+  * [Update time complexity of some HashSet functions](https://github.com/haskell-unordered-containers/unordered-containers/pull/568)
+  * [Update instructions for code inspection](https://github.com/haskell-unordered-containers/unordered-containers/pull/567)
+
+* Other changes:
+  * [Drop support for GHC < 8.10](https://github.com/haskell-unordered-containers/unordered-containers/pull/510)
+  * [Address deprecation warnings and other warnings](https://github.com/haskell-unordered-containers/unordered-containers/pull/512)
+  * [Introduce `ShiftedHash`](https://github.com/haskell-unordered-containers/unordered-containers/pull/529)
+  * [New "fine-grained" benchmarks](https://github.com/haskell-unordered-containers/unordered-containers/pull/526)
+  * [Make it compile with MicroHs](https://github.com/haskell-unordered-containers/unordered-containers/pull/553). Thanks, @augustss!
+  * [Remove redundant `Eq` constraints](https://github.com/haskell-unordered-containers/unordered-containers/pull/558)
+  * [Refactor `delete`](https://github.com/haskell-unordered-containers/unordered-containers/pull/571)
+  * [`difference[With]`: Undo constraint relaxation](https://github.com/haskell-unordered-containers/unordered-containers/pull/573)
+
+[0.2.21]: https://github.com/haskell-unordered-containers/unordered-containers/compare/v0.2.20.1...v0.2.21
+
 ## [0.2.20.1] - October 2025
 
 * [Fix infinite loop in `isSubmapOf[By]` / `isSubsetOf` on 32-bit platforms](https://github.com/haskell-unordered-containers/unordered-containers/pull/501).