Add AppVeyor (Windows CI) badge to README.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=237295321

README.md

@@ -1,6 +1,7 @@
 **LevelDB is a fast key-value storage library written at Google that provides an ordered mapping from string keys to string values.**
 
 [![Build Status](https://travis-ci.org/google/leveldb.svg?branch=master)](https://travis-ci.org/google/leveldb)
+[![Build status](https://ci.appveyor.com/api/projects/status/g2j5j4rfkda6eyw5/branch/master?svg=true)](https://ci.appveyor.com/project/pwnall/leveldb)
 
 Authors: Sanjay Ghemawat (sanjay@google.com) and Jeff Dean (jeff@google.com)