Reference to nginx docs for PROXY_CACHE_USE_STALE

Author: mhbahmani

docs/getting_started.md

@@ -39,7 +39,7 @@ running as a Container or as a Systemd service.
 | `PROXY_CACHE_VALID_OK`                | No        |                                            | `1h`      | Sets caching time for response code 200 and 302                                                                                                                                                                                                                                                                                                                                                                                                              |
 | `PROXY_CACHE_VALID_NOTFOUND`          | No        |                                            | `1m`      | Sets caching time for response code 404                                                                                                                                                                                                                                                                                                                                                                                                                      |
 | `PROXY_CACHE_VALID_FORBIDDEN`         | No        |                                            | `30s`     | Sets caching time for response code 403                                                                                                                                                                                                                                                                                                                                                                                                                      |
-| `PROXY_CACHE_USE_STALE`               | No        |                                            | `error timeout http_500 http_502 http_503 http_504` | Sets conditions under which stale cached data can be used.                                                                                                                                                                                                                                                                                                                                                 |
+| `PROXY_CACHE_USE_STALE`               | No        |                                            | `error timeout http_500 http_502 http_503 http_504` | Sets conditions under which stale cached data can be used. See [here](https://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_cache_valid) for more detailes                                                                                                                                                                                                                                               |
 | `PROVIDE_INDEX_PAGE`                  | No        | `true`, `false`                            | `false`   | Flag which returns the index page if there is one when requesting a directory.                                                                                                                                                                                                                                                                                                                                                                               |
 | `JS_TRUSTED_CERT_PATH`                | No        |                                            |           | Enables the `js_fetch_trusted_certificate` directive when retrieving AWS credentials and sets the path (on the container) to the specified path                                                                                                                                                                                                                                                                                                              |
 | `HEADER_PREFIXES_TO_STRIP`            | No        |                                            |           | A list of HTTP header prefixes that exclude headers from client responses. List should be specified in lower-case and a semicolon (;) should be used to as a delimiter between values. For example: x-goog-;x-something-. Headers starting with x-amz- will be stripped by default for security reasons unless explicitly added in HEADER_PREFIXES_ALLOWED.                                                                                              |