meta: fix link to release

Author: tbaliance

README.md

@@ -2,7 +2,7 @@
 and .pptx).  
 
 [![Build Status](https://travis-ci.org/baliance/gooxml.svg?branch=master)](https://travis-ci.org/baliance/gooxml)
-[![GitHub (pre-)release](https://img.shields.io/github/release/baliance/gooxml/all.svg)]()
+[![GitHub (pre-)release](https://img.shields.io/github/release/baliance/gooxml/all.svg)](https://github.com/baliance/gooxml/releases)
 [![License: AGPL v3](https://img.shields.io/badge/License-Dual%20AGPL%20v3/Commercial-blue.svg)](https://www.gnu.org/licenses/agpl-3.0)
 [![GoDoc](https://godoc.org/baliance.com/gooxml?status.svg)](https://godoc.org/baliance.com/gooxml)
 [![Go Report Card](https://goreportcard.com/badge/baliance.com/gooxml)](https://goreportcard.com/report/baliance.com/gooxml)