Fix links to README and CONTRIBUTING in docs/index.rst (#260)

diff --git a/docs/index.rst b/docs/index.rst
index af2c804..56e3eca 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -57,7 +57,7 @@
 details, see `LICENSE`_
 
 .. _LICENSE:
-    https://github.com/GoogleCloudPlatform/google-auth-library-python/LICENSE
+    https://github.com/GoogleCloudPlatform/google-auth-library-python/blob/master/LICENSE
 
 Contributing
 ------------
@@ -66,4 +66,4 @@
 for details.
 
 .. _contributing:
-    https://github.com/GoogleCloudPlatform/google-auth-library-python/CONTRIBUTING.rst
+    https://github.com/GoogleCloudPlatform/google-auth-library-python/blob/master/CONTRIBUTING.rst