add compatibility check badges to README (#346)

diff --git a/README.rst b/README.rst
index 00dbfef..c171f95 100644
--- a/README.rst
+++ b/README.rst
@@ -1,7 +1,7 @@
 Google Auth Python Library
 ==========================
 
-|build| |docs| |pypi|
+|build| |docs| |pypi| |compat_check_pypi| |compat_check_github|
 
 This library simplifies using Google's various server-to-server authentication
 mechanisms to access Google APIs.
@@ -12,6 +12,10 @@
    :target: https://google-auth.readthedocs.io/en/latest/
 .. |pypi| image:: https://img.shields.io/pypi/v/google-auth.svg
    :target: https://pypi.python.org/pypi/google-auth
+.. |compat_check_pypi| image:: https://python-compatibility-tools.appspot.com/one_badge_image?package=google-auth
+   :target: https://python-compatibility-tools.appspot.com/one_badge_target?package=google-auth
+.. |compat_check_github| image:: https://python-compatibility-tools.appspot.com/one_badge_image?package=git%2Bgit%3A//github.com/googleapis/google-auth-library-python.git
+   :target: https://python-compatibility-tools.appspot.com/one_badge_target?package=git%2Bgit%3A//github.com/googleapis/google-auth-library-python.git
 
 Installing
 ----------