Add Appveyor CI badge

Author: elprans

README.rst

@@ -4,6 +4,9 @@ asyncpg -- A fast PostgreSQL Database Client Library for Python/asyncio
 .. image:: https://travis-ci.org/MagicStack/asyncpg.svg?branch=master
     :target: https://travis-ci.org/MagicStack/asyncpg
 
+.. image:: https://ci.appveyor.com/api/projects/status/github/MagicStack/asyncpg?branch=master
+    :target: https://ci.appveyor.com/project/elprans/asyncpg
+
 .. image:: https://img.shields.io/pypi/v/asyncpg.svg
     :target: https://pypi.python.org/pypi/asyncpg