Increment version number to 3.1.0 and add contributor

Author: nanxstats

DESCRIPTION

@@ -1,12 +1,13 @@
 Package: ggsci
 Type: Package
 Title: Scientific Journal and Sci-Fi Themed Color Palettes for 'ggplot2'
-Version: 3.0.3
+Version: 3.1.0
 Authors@R: c(
     person("Nan", "Xiao", email = "me@nanx.me", role = c("aut", "cre"),
            comment = c(ORCID = "0000-0002-0250-5673")),
     person("Joshua", "Cook", email = "joshuacook0023@gmail.com", role = "ctb"),
     person("Clara", "Jégousse", email = "cat3@hi.is", role = "ctb"),
+    person("Hui", "Chen", email = "huichen@zju.edu.cn", role = "ctb"),
     person("Miaozhu", "Li", email = "miaozhu.li@duke.edu", role = "ctb")
     )
 Maintainer: Nan Xiao <me@nanx.me>