DOC: Don't forget xcb qt5 plugin... [gen-appimage]

README.md

@@ -39,6 +39,7 @@ To build Klkt, you will need:
 - libnotify, the developement package
 - glib-2.0, the developpement package
 - qmake and Qt5 developpement libraries (Qt5Core Qt5Gui Qt5Network Qt5Widgets)
+- don't forget the qt5dxcb-plugin (xcb plugin for qt5) when compiling
 
 You will also need the following executables, with their path setted in the configuration file: