make links to code / paper clearer (#146)

Author: sbfnk

README.Rmd

@@ -22,6 +22,13 @@ set.seed(111)
 [![License: MIT](https://img.shields.io/badge/License-MIT-blue.svg)](https://opensource.org/licenses/MIT)
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+_{{ packagename }}_ is an R package that provides methods to simulate infectious
+disease transmission in the presence of contact tracing. It was
+initially developed to support a paper written in early 2020 to assess
+the feasibility of controlling
+COVID-19; see the [companion analysis code](https://github.com/cmmid/ringbp) and 
+[paper](https://doi.org/10.1016/S2214-109X(20)30074-7) for more details.
+
 _{{ packagename }}_ is an R package that provides methods to simulate infectious disease transmission in the presence of contact tracing.
 It was initially developed to support a paper written in early 2020 to assess the [feasibility of controlling COVID-19](https://github.com/cmmid/ringbp).
 For more details on the methods implemented here, see the associated [paper](https://doi.org/10.1016/S2214-109X(20)30074-7).