adding cache hit note

Author: jkyberneees

README.md

@@ -100,7 +100,9 @@ res.setHeader('cache-control', 'private, no-cache, max-age=300')
 res.setHeader('etag', '1')
 ```
 This will enable browser clients to keep a copy of the cache on their side, but still being forced to validate 
-the cache state on the server before using the cached response, therefore supporting gateway based cache invalidation.  
+the cache state on the server before using the cached response, therefore supporting gateway based cache invalidation. 
+
+> NOTE: In order to fetch the generated `Cache-Control` and `ETag` headers, there have to be at least cache hit.
 
 ### Invalidating caches 
 Services can easily expire cache entries on demand, i.e: when the data state changes. Here we use the `x-cache-expire` header to indicate the cache entries to expire using a matching pattern: