Update CHANGELOG.md

CHANGELOG.md

+## 2023-05-12
+
+* Added feature si:quantityTypeQUDT (identification of kinds of quantities)
+* Extended and improved structures for representation of measurement uncertainties
+* Added feature si:standardMU (standard measurement uncertainty)
+* Added feature NaN values (placeholders)
+* Added unit operator /per 
+* Improved documentation of k value and distribution usage
+* Added feature si:significantDigit
+* Cleaned up XSD
+
+* Update of Wiki with page providing all unit identifiers [>> units Wiki](wiki/doc/UnitSystem.md)
+
 ## 2022-11-30
 
 * Update of units representation system with new SI prefixes and dimension one units