Added sentence about lightweight models

Author: aturon

active/0000-remove-runtime.md

@@ -132,7 +132,8 @@ several problems in practice.
   style of lightweight tasks is used in Servo, but also shows up in
   [java.util.concurrent's exectors](http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Executors.html)
   and [Haskell's par monad](https://hackage.haskell.org/package/monad-par),
-  among many others.
+  among many others. These lighter weight models do not fit into the current
+  runtime system.
 
   On the other hand, green threading systems designed explicitly to support I/O
   may also want to provide low-level access to the underlying event loop -- an