Add Kotlin's datetime library to the list of projects using this (How…

…ardHinnant#593)

README.md

@@ -78,5 +78,6 @@ cmake --build . --target testit # Consider '-- -j4' for multithreading
 * https://github.com/valhalla
 * https://github.com/siodb/siodb
 * https://github.com/KomodoPlatform/atomicDEX-Pro
+* https://github.com/Kotlin/kotlinx-datetime
 
 If you would like your project (or product) on this list, just let me know.