docs: add minimum golang version in README

Author: lucab

README.md

@@ -2,6 +2,8 @@
 
 [![Build Status](https://travis-ci.org/coreos/go-systemd.png?branch=master)](https://travis-ci.org/coreos/go-systemd)
 [![godoc](https://godoc.org/github.com/coreos/go-systemd?status.svg)](http://godoc.org/github.com/coreos/go-systemd)
+![minimum golang 1.5](https://img.shields.io/badge/golang-1.5%2B-orange.svg)
+
 
 Go bindings to systemd. The project has several packages: