New text about "from __future__ import generators".

pep-0255.txt

@@ -125,10 +125,15 @@ Specification:  Yield
         yield_stmt:    "yield" expression_list
 
     "yield" is a new keyword, so a future statement[8] is needed to phase
-    this in.  [XXX spell this out -- but new keywords have ripple effects
-    across tools too, and it's not clear this can be forced into the future
-    framework at all -- it's not even clear that Python's parser alone can
-    be taught to swing both ways based on a future stmt]
+    this in:  in the initial release, a module desiring to use generators
+    must include the line
+
+        from __future__ import generators
+
+    near the top (see PEP 236[8]) for details).  Modules using the
+    identifier "yield" without a future statement will trigger warnings.
+    In the following release, yield will be a language keyword and the
+    future statement will no longer be needed.
 
     The yield statement may only be used inside functions.  A function that
     contains a yield statement is called a generator function.  A generator