message: Update README.md with multiple windows
diff: @@ -38,6 +38,7 @@ The primary developer is [Simon Edwards](mailto:simon@simonzone.com). Extraterm
  * Text zoom
  * Unicode support
  * Color emoji
  +* Multiple windows
  * [MIT license](LICENSE.txt)
  * [...much more to come](https://github.com/sedwards2009/extraterm/issues/30)
  