Add copyright holder role

DESCRIPTION

@@ -4,8 +4,8 @@ Title: Shorten the Distance from Data Visualization Idea to Actual Plot
 Version: 0.1.0
 Authors@R: c(
     person("Thomas", "Neitmann", , "[email protected]", c("aut", "cre")),
-    person("Julia", "Silge", role = "ctb"),
-    person("David", "Robinson", role = "ctb")
+    person("Julia", "Silge", role = c("ctb", "cph")),
+    person("David", "Robinson", role = c("ctb", "cph"))
     )
 Description: Streamline the creation of common charts by taking care of a lot of
     data preprocessing and plot customization for the user. Provides a