Add package level documentation and citation information

Change-Id: I109311d37747d7c65bd7db893f10e25ebe432e3a
diff --git a/NAMESPACE b/NAMESPACE
index 0ac2973..bcf44e9 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -13,3 +13,4 @@
 importFrom(httr2,request)
 importFrom(httr2,resp_body_json)
 importFrom(magrittr,"%>%")
+importFrom(utils,citation)