docs: add dynamic replacements

README.md

@@ -43,6 +43,14 @@ app.use(serveStatic('public'));
 
 app.listen(3000);
 ```
+### Dynamic replacements
+
+You might also replace dynamically with access to all the content of [Request](https://expressjs.com/de/api.html#req) and [Response](https://expressjs.com/de/api.html#res):
+
+```
+stringReplace({
+    '$host' => (req, res) => 'your host ' + req.hostname
+});
 
 ## Configuration
 
@@ -61,4 +69,4 @@ app.use(stringReplace({
 
 The default regex is `/^text\/|^application\/json$|^application\/xml$/`, which will match `text/*`, `application/json`, and `application/xml`. Any response with a Content-Type header that doesn't match the regex is ignored and passed-through without modification. 
 
-Also any response without a Content-Type header is ignored and passed-through without any modification.
+Also any response without a Content-Type header is ignored and passed-through without any modification.