Add release version tag

Author: bitwiseman

README.md

@@ -1,3 +1,5 @@
+[![Sonatype Nexus (Releases)](https://img.shields.io/nexus/r/org.kohsuke/github-api?server=https%3A%2F%2Foss.sonatype.org)](https://mvnrepository.com/artifact/org.kohsuke/github-api)
+
 Java API for GitHub
 
 See https://github-api.kohsuke.org/ for more details