Make shiny frequency curves run as a proper R demo

Change-Id: I75dbfd97c98cd586f012e4d6409ed71d7745a201
diff --git a/demo/shiny-frequency-curves.R b/demo/shiny-frequency-curves.R
new file mode 100644
index 0000000..ea8b43d
--- /dev/null
+++ b/demo/shiny-frequency-curves.R
@@ -0,0 +1 @@
+shiny::runApp(system.file("shiny-apps", "frequency_curves", package="RKorAPClient"))