Update CHANGELOG.md

Author: nashby

CHANGELOG.md

@@ -1,6 +1,7 @@
 ## master ##
   * Store mail location in `Mail::Message` object (thanks [Jonathan Chan](https://github.com/jonmchan))
   * Drop Ruby 2 support. Support only Ruby 3.0+
+  * Remove `nkf` gem dependency.
 
 ## 1.8.1 ##
  * Fix duplication of Rails tasks caused by LetterOpener's rake task file. (thanks [zarqman](https://github.com/zarqman))