Add modification warning to templated files
diff --git a/templates/gRPC-ProtoRPC.podspec.template b/templates/gRPC-ProtoRPC.podspec.template
index 4b507d2..5d7d90d 100644
--- a/templates/gRPC-ProtoRPC.podspec.template
+++ b/templates/gRPC-ProtoRPC.podspec.template
@@ -1,5 +1,11 @@
 %YAML 1.2
 --- |
+  # This file has been automatically generated from a template file.
+  # Please make modifications to
+  # `templates/gRPC-ProtoRPC.podspec.template` instead. This file can be
+  # regenerated from the template by running
+  # `tools/buildgen/generate_projects.sh`.
+
   # Copyright 2015, Google Inc.
   # All rights reserved.
   #