hep: add appveyor badge

Author: sbinet

README.md

@@ -2,6 +2,7 @@ hep
 ===
 
 [![Build Status](https://travis-ci.org/go-hep/hep.svg?branch=master)](https://travis-ci.org/go-hep/hep)
+[![Build status](https://ci.appveyor.com/api/projects/status/qnnp26vv2c71f560?svg=true)](https://ci.appveyor.com/project/sbinet/hep)
 [![codecov](https://codecov.io/gh/go-hep/hep/branch/master/graph/badge.svg)](https://codecov.io/gh/go-hep/hep)
 [![Go Report Card](https://goreportcard.com/badge/github.com/go-hep/hep)](https://goreportcard.com/report/github.com/go-hep/hep)
 [![GoDoc](https://godoc.org/go-hep.org/x/hep?status.svg)](https://godoc.org/go-hep.org/x/hep)