Add build status badges to README

Author: oschulz

README.md

@@ -1 +1,5 @@
 # ArraysOfArrays.jl
+
+[![Build Status](https://travis-ci.com/oschulz/ArraysOfArrays.jl.svg?branch=master)](https://travis-ci.com/oschulz/ArraysOfArrays.jl)
+[![Build Status](https://ci.appveyor.com/api/projects/status/github/oschulz/ArraysOfArrays.jl?branch=master&svg=true)](https://ci.appveyor.com/project/oschulz/ArraysOfArrays-jl)
+[![Codecov](https://codecov.io/gh/oschulz/ArraysOfArrays.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/oschulz/ArraysOfArrays.jl)