[gnome-builder/wip/gtk4-port] plugins/symbol-tree: add note about porting

[Christian Hergert <chergert src gnome org>]

commit bbf3312e7e48289d5e6246a550eeee35e009d594
Author: Christian Hergert <chergert redhat com>
Date:   Tue Apr 19 10:36:18 2022 -0700

    plugins/symbol-tree: add note about porting

 src/plugins/symbol-tree/TODO.md | 6 ++++++
 1 file changed, 6 insertions(+)
---
diff --git a/src/plugins/symbol-tree/TODO.md b/src/plugins/symbol-tree/TODO.md
new file mode 100644
index 000000000..e160d288f
--- /dev/null
+++ b/src/plugins/symbol-tree/TODO.md
@@ -0,0 +1,6 @@
+# GTK 4 Port
+
+These should be switched to using GListModel for symbols and GtkListView as a tree.
+When searching, we can instead use a Flatten list model and search across that.
+The menu button should be added into the frame headerbar, but we'll need to do something different when tabs 
are in use.
+