checked nested menus item, added a comment about menu null termination

README.md

@@ -59,6 +59,8 @@ struct tray_menu {
 
   void (*cb)(struct tray_menu *);
   void *context;
+
+  struct tray_menu *submenu;
 };
 ```
 
@@ -69,6 +71,9 @@ struct tray_menu {
 
 All functions are meant to be called from the UI thread only.
 
+Menu arrays must be terminated with a NULL item, e.g. the last item in the
+array must have text field set to NULL.
+
 ## Roadmap
 
 * [x] Cross-platform tray icon
@@ -76,7 +81,7 @@ All functions are meant to be called from the UI thread only.
 * [x] Separators in the menu
 * [x] Disabled/enabled menu items
 * [x] Checked/unchecked menu items
-* [ ] Nested menus
+* [x] Nested menus
 * [ ] Icons for menu items
 * [ ] Rewrite ObjC code in C using ObjC Runtime (now ObjC code breaks many linters and static analyzers)
 * [ ] Call GTK code using dlopen/dlsym (to make binaries run safely if Gtk libraries are not available)