Update CHANGELOG for v1.0.0

***NO_CI***

Author: nfbot

CHANGELOG.md

@@ -1,8 +1,8 @@
 # Changelog
 
-## [**Changes available only in 'Preview' NuGet packages:**](https://github.com/nanoframework/nanoFramework.WebServer/tree/HEAD)
+## [v1.0.0-preview.78](https://github.com/nanoframework/nanoFramework.WebServer/tree/v1.0.0-preview.78) (2021-05-19)
 
-[Full Changelog](https://github.com/nanoframework/nanoFramework.WebServer/compare/v1.0.0-preview.76...HEAD)
+[Full Changelog](https://github.com/nanoframework/nanoFramework.WebServer/compare/v1.0.0-preview.76...v1.0.0-preview.78)
 
 **Documentation and other chores:**