Add badge to readme showing latest stable release

README.md

 # Sonic Robo Blast 2
+[![latest release](https://badgen.net/github/release/STJr/SRB2/stable)](https://github.com/STJr/SRB2/releases/latest)
 
 [![Build status](https://ci.appveyor.com/api/projects/status/399d4hcw9yy7hg2y?svg=true)](https://ci.appveyor.com/project/STJr/srb2)
 [![Build status](https://travis-ci.org/STJr/SRB2.svg?branch=master)](https://travis-ci.org/STJr/SRB2)