Added prependPrefix to Group class & updated documentation.

src/Pecee/SimpleRouter/Route/IGroupRoute.php

@@ -53,6 +53,14 @@ interface IGroupRoute extends IRoute
      */
     public function setDomains(array $domains): self;
 
+    /**
+     * Prepend url
+     *
+     * @param string $url
+     * @return static
+     */
+    public function prependUrl(string $url): self;
+
     /**
      * Set prefix that child-routes will inherit.
      *