Mention ReactiveExample in release notes

Terminal.Gui/Terminal.Gui.csproj

@@ -24,6 +24,7 @@
     <PackageReleaseNotes>
       v1.00 -
       * If StatusBar.Visible is set to false, TopLevel resizes correctly enabling hiding/showing of a StatusBar. UICatalog demonstrates.
+      * New sample/demo app - ReactiveExample - Shows how to use reactive extensions and ReactiveUI with gui.cs.
       
       v0.90 - "Feature Complete" pre-release of Terminal.Gui (aka gui.cs) 1.0. This release is a signficant upgrade from the previous published release (0.81). Most of the major changes and bug fixes are listed below. NOTE: This release includes breaking changes to the API; we will strive to avoid any more breaking changes before 1.0.