adding cache-control feature documentation

Author: jkyberneees

README.md

@@ -77,7 +77,32 @@ service.get('/numbers', (req, res) => {
 })
 ```
 
-### Invalidating caches
+### Caching on the browser side (304 status codes)
+> From version `1.2.x` you can also use the HTTP compatible `Cache-Control` header: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Cache-Control
+When using the Cache-Control header, you can omit the custom `x-cache-timeout` header as the timeout can be passed using the `max-age` directive. 
+
+#### Direct usage: 
+```js 
+res.setHeader('cache-control', `private, no-cache, max-age=${60 * 5}`) 
+res.setHeader('etag', '1')
+
+res.end('5 minutes cacheable content here....')
+```
+
+#### Indirect usage:
+When using:
+```js
+res.setHeader('x-cache-timeout', '5 minutes')
+```
+The middleware will now transparently generate default `Cache-Control` and `ETag` headers as described below:
+```js 
+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.  
+
+### 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:
 ```js
 res.setHeader('x-cache-expire', '*/numbers')