Updated changelog

Author: jfalcou

docs/changelog.md

@@ -1,3 +1,17 @@
+# Version 2.1 - Charming Chrysoprase
+
+## What's Changed
+
+### Bug Fixes
+ - Improve `kumi::cat` implementation to perform `O(N)` copies.
+ - Use include guards to prevent issue when different project use KUMI concurrently.
+
+### New Features
+ - Added the `kumi::sized_product_type_or_more` concept
+ - Implements `kumi::to_ref` to construct a tuple of references form a reference to a tuple.
+
+**Full Changelog**: https://github.com/jfalcou/kumi/compare/v2.0...v2.1
+
 # Version 2.0 - Bedazzling Beryl
 
 ## What's Changed