Add release badge

Author: lspgn

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://travis-ci.org/cloudflare/gortr.svg?branch=master)](https://travis-ci.org/cloudflare/gortr)
 [![GoDoc](https://godoc.org/github.com/cloudflare/gortr?status.svg)](https://pkg.go.dev/github.com/cloudflare/gortr)
+![GitHub release (latest by date)](https://img.shields.io/github/v/release/cloudflare/gortr)
 
 GoRTR is an open-source implementation of RPKI to Router protocol (RFC 6810) using the [the Go Programming Language](http://golang.org/).