Add example on how to enable jwt for specific path

Add example on how to enable jwt for specific path, as compared to _disabling_ for a specific path which was the only example shown before.

README.md

@@ -71,7 +71,13 @@ jwt({
 });
 ```
 
-Optionally you can make some paths unprotected as follows:
+To only protect specific paths (e.g. beginning with `/api`), use [express router](https://expressjs.com/en/4x/api.html#app.use) like so:
+
+```javascript
+app.use('/api', jwt({ secret: 'shhhhhhared-secret', algorithms: ['HS256']}));
+```
+
+Or, the other way around, if you want to to make some paths unprotected as follows:
 
 ```javascript
 app.use(