FFM : add note about upcalls

Author: pquiring

docs/ffm.txt

@@ -12,3 +12,8 @@ JF approach is to use JNI to pin arrays to avoid the excess copying back and for
 
 Maybe the Valhalla project and value types may offer better performance in the future but still have no idea how Valhalla will work with the native systems (FFM/JNI).
 See valhalla.txt for more info.
+
+Notes:
+  - FFM upcalls (allows C code to call Java) are somewhat expensive so instead of creating them for each native invocation static upcalls are used instead
+    - in order for static upcalls to call virtual methods ThreadLocal storage is used to track which object invoked the native method
+    - one caveat to this is that callback methods can NOT call another native method