Clarify deprecation of EXTRA_QT_PLUGINS and update README.md #235

Job	Run time
aarch64 appimage	7m 21s
i386 appimage	7m 3s
x86_64 coverage	5m 38s
x86_64 appimage	5m 39s
armhf appimage	8m 46s
Create release and upload artifacts	8s
	34m 35s

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