Add used-by-Python comments to top-level files
diff --git a/tox.ini b/tox.ini
index 8bb5fc2..f89f34e 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,3 +1,4 @@
+# GRPC Python tox (test environment) settings
 [tox]
 skipsdist = true
 envlist = py27