calculate-cache.sh: mention response cache file

Signed-off-by: Norio Nomura <[email protected]>

hack/calculate-cache.sh

@@ -11,6 +11,7 @@
 # 1. extracts `runs_on` and `template` from workflow file (.github/workflows/test.yml)
 # 2. check each template for image and nerdctl
 # 3. detect size of image and nerdctl (responses from remote are cached for faster iteration)
+#    save the response in .calculate-cache-response-cache.yaml
 # 4. print content size, actual cache size (if available), by cache key
 #
 # The major differences for reducing cache usage are as follows: