add preprint link

index.md

@@ -2,7 +2,9 @@
 [![Coverage Status](https://codecov.io/github/waldronlab/HGNChelper/coverage.svg?branch=master)](https://codecov.io/github/waldronlab/HGNChelper?branch=master)
 [![](https://cranlogs.r-pkg.org/badges/HGNChelper)](https://cran.r-project.org/package=HGNChelper)
 
-# HGNChelper: Identify and correct invalid gene symbols
+# HGNChelper: Identification and correction of invalid gene symbols for human and mouse
+
+A [pre-print is available on bioRxiv](https://www.biorxiv.org/content/10.1101/2020.09.16.300632v2). 
 
 ## Why HGNChelper?
 
@@ -29,4 +31,4 @@ a little more pleasant to bioinformaticians.
 * See the [Reference](reference/index.html) for function documentation.
 * See the [GitHub](https://github.com/waldronlab/HGNChelper) page for
   creating your own package update with the most current gene lists,
-  rather than updating within R as described in the vignette
+  rather than updating within R as described in the vignette