added comment about setting cacheable flag.
diff --git a/doc/interop-test-descriptions.md b/doc/interop-test-descriptions.md
index 62d3670..19947b3 100644
--- a/doc/interop-test-descriptions.md
+++ b/doc/interop-test-descriptions.md
@@ -70,6 +70,10 @@
Note that client adds a `x-user-ip` header with value `1.2.3.4` to the request.
This is done since some proxys such as GFE will not cache requests from
localhost.
+Note also that the client request needs to marked as cacheable. For now this is
+achieved by setting the cacheable flag in the request context to 'true'.Longer
+term this will be automatically set via method options specified in the proto
+file.
Server features:
* [CacheableUnaryCall][]
