Add license notes on Highcharts

Change-Id: I4b1fe666d11a8f3abd3c450bbaa44c7eba97a0c8
diff --git a/R/highcharter-helper.R b/R/highcharter-helper.R
index ad2168c..6601655 100644
--- a/R/highcharter-helper.R
+++ b/R/highcharter-helper.R
@@ -136,3 +136,11 @@
   }
   hc
 }
+
+.onAttach <- function(libname = find.package("RKorAPClient"),
+                      pkgname = "RKorAPClient") {
+  packageStartupMessage(
+    "If you intend to use the Highcharts plot options, please note that Highcharts (www.highcharts.com) is a Highsoft software product which is not free for commercial and governmental use."
+  )
+}
+