Add disclaimer for multi side-menus support

Author: d4vidi

lib/Mock/Stores/LayoutStore.ts

@@ -78,6 +78,12 @@ const setters = remx.setters({
     getters.getLayoutById(layout.nodeId).selectedIndex = index;
   },
   openSideMenu(layout: SideMenuNode) {
+    if (state.sideMenu) {
+      throw new Error(
+        'A side-menu is already open; Mocked-testing of multiple side-menu scenarios is not supported yet.' +
+          ' You can submit a request in https://github.com/wix/react-native-navigation/issues/new/choose.'
+      );
+    }
     state.sideMenu = layout;
   },
   closeSideMenu(_layout: SideMenuNode) {