Clarify deprecation of EXTRA_QT_PLUGINS and update README.md #238

Job	Run time
x86_64 coverage	7m 54s
x86_64 appimage	6m 41s
i386 appimage	7m 57s
aarch64 appimage	7m 12s
armhf appimage	8m 0s
Create release and upload artifacts	13s
	37m 57s

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