ci: add VSTS build badge to README

(cherry picked from commit a1ae41b8)

README.md

@@ -3,6 +3,7 @@ libgit2 - the Git linkable library
 
 [![Travis Build Status](https://secure.travis-ci.org/libgit2/libgit2.svg?branch=master)](http://travis-ci.org/libgit2/libgit2)
 [![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/xvof5b4t5480a2q3/branch/master?svg=true)](https://ci.appveyor.com/project/libgit2/libgit2/branch/master)
+[![VSTS Build Status](https://libgit2.visualstudio.com/libgit2/_apis/build/status/libgit2)](https://libgit2.visualstudio.com/libgit2/_build/latest?definitionId=7)
 [![Coverity Scan Build Status](https://scan.coverity.com/projects/639/badge.svg)](https://scan.coverity.com/projects/639)
 
 `libgit2` is a portable, pure C implementation of the Git core methods