Add dependency on R (>= 3.5.0)

Change-Id: I72dcbcd23a7b60aa81fd6bf2dfe07d01c9da66d1
diff --git a/DESCRIPTION b/DESCRIPTION
index 55edfaf..f259e53 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -5,6 +5,7 @@
 Author: KorAP-Team @ IDS Mannheim
 Maintainer: KorAP-Team <korap@ids-mannheim.de>
 Description: A simple package that makes KorAP's web serice API accessible from R.
+Depends: R (>= 3.5.0)
 License: BSD_2_clause + file LICENSE
 Encoding: UTF-8
 LazyData: false