Also ship robots.txt in the source package (#5811)

Makefile

 
 source-package:
 	mkdir -p source_package/jitsi-meet/css && \
-	cp -r *.js *.html connection_optimization favicon.ico fonts images libs static sounds LICENSE lang source_package/jitsi-meet && \
+	cp -r *.js *.html resources/*.txt connection_optimization favicon.ico fonts images libs static sounds LICENSE lang source_package/jitsi-meet && \
 	cp css/all.css source_package/jitsi-meet/css && \
 	(cd source_package ; tar cjf ../jitsi-meet.tar.bz2 jitsi-meet) && \
 	rm -rf source_package