Added important info on how to get icon themes working (#323)

[ci skip]

README.md

@@ -131,6 +131,12 @@ find $HOME/build-*-*_Qt_* \( -name "moc_*" -or -name "*.o" -or -name "qrc_*" -or
 
 Alternatively, you could use `$DESTDIR`.
 
+#### Adding icon and icon theme support
+
+To enable icon and icon theme support you must add `iconengines` as an extra Qt plugin while running `linuxdeployqt`.
+
+`-extra-plugins=iconengines`
+
 #### Adding extra Qt plugins 
 
 If you want aditional plugins which the tool doesn't deploy, for a variety of reasons, you can use the -extra-plugins argument and include a list of plugins separated by a comma.