update sample code

README.md

@@ -57,6 +57,18 @@ r3_tree_insert_pathl(n ,"/post/{id}", strlen("/post/{id}") , &route_data );
 
 r3_tree_insert_pathl(n, "/user/{id:\\d+}", strlen("/user/{id:\\d+}"), &route_data );
 
+
+// if you want to cache error, you may call the extended path function for insertion
+int data = 10;
+char *errstr = NULL;
+node *ret = r3_tree_insert_pathl_ex(n, "/foo/{name:\\d{5}", strlen("/foo/{name:\\d{5}"), NULL, &data, &errstr);
+if (ret == NULL) {
+    // failed insertion
+    printf("error: %s\n", errstr);
+    free(errstr); // errstr is created from `asprintf`, so you have to free it manually.
+}
+
+
 // let's compile the tree!
 char *errstr = NULL;
 int err = r3_tree_compile(n, &errstr);