add AppVeyor badge

Author: reverbc

README.md

@@ -2,6 +2,7 @@
 
 [![PyPI version fury.io](https://badge.fury.io/py/pylint-pytest.svg)](https://pypi.python.org/pypi/pylint-pytest/)
 [![Travis CI](https://travis-ci.org/reverbc/pylint-pytest.svg?branch=master)](https://travis-ci.org/reverbc/pylint-pytest)
+[![AppVeyor](https://ci.appveyor.com/api/projects/status/github/reverbc/pylint-pytest?branch=master&svg=true)](https://ci.appveyor.com/project/reverbc/pylint-pytest)
 
 A Pylint plugin to suppress pytest-related false positives.