Readme tweak

Author: chambart

README.md

@@ -10,4 +10,4 @@ assert(Nullable_array.get 1 = Some 4);
 ```
 
 [![Build Status (Travis)](https://travis-ci.org/chambart/ocaml-nullable-array.svg)](https://travis-ci.org/chambart/ocaml-nullable-array)
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+[![Build Status (Appveyor)](https://ci.appveyor.com/api/projects/status/github/chambart/ocaml-nullable-array?svg=true)](https://ci.appveyor.com/project/chambart/ocaml-nullable-array)