Add used-by-Python comments to top-level files
diff --git a/requirements.txt b/requirements.txt
index 06516ee..9d00274 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,3 +1,4 @@
+# GRPC Python setup requirements
 enum34>=1.0.4
 futures>=2.2.0
 cython>=0.23