Add build badges to README

Author: nicoddemus

README.rst

@@ -2,6 +2,15 @@
 Plugin registration and hook calling for Python
 ===============================================
 
+.. image:: https://img.shields.io/pypi/v/pluggy.svg
+    :target: https://pypi.python.org/pypi/pluggy
+.. image:: https://img.shields.io/pypi/pyversions/pluggy.svg
+    :target: https://pypi.python.org/pypi/pluggy
+.. image:: https://img.shields.io/travis/pytest-dev/pluggy/master.svg
+    :target: https://travis-ci.org/pytest-dev/pluggy
+.. image:: https://img.shields.io/appveyor/ci/pytestbot/pluggy/master.svg
+    :target: https://ci.appveyor.com/project/pytestbot/pluggy
+
 This is the plugin manager as used by pytest but stripped
 of pytest specific details.