Add TODO items

TODO

diff --git a/TODO b/TODO
index 4b8db97..98391ab 100644
--- a/TODO
+++ b/TODO
@@ -1,3 +1,6 @@
+- head table access cleanup (div by zero now!)
+- cache various expensive scale computation
+
 - GNOME Bug 612402 - (hb-arm) HarfBuzz compilation fix for arm
 
 - Make sure LangSys default feature is only applied once...