Add trayhost package to GUI section.

Package trayhost is a cross-platform Go library to place an icon in the host operating system's taskbar.

README.md

@@ -408,6 +408,7 @@ Please take a quick gander at the [contribution guidelines](https://github.com/a
 * [gotk3](https://github.com/gotk3/gotk3) - Go bindings for GTK3.
 * [gxui](https://github.com/google/gxui) - A Go cross platform UI library.
 * [sciter](https://github.com/oskca/sciter) - Go bindings for Sciter: the Embeddable HTML/CSS/script engine for modern desktop UI development.
+* [trayhost](https://github.com/shurcooL/trayhost) - Cross-platform Go library to place an icon in the host operating system's taskbar.
 * [ui](https://github.com/andlabs/ui) - Platform-native GUI library for Go.
 * [walk](https://github.com/lxn/walk) - Windows application library kit for Go.