Make it clear that 1.0 is not released yet

docs/source/whatsnew/version1.0.rst

@@ -5,6 +5,10 b''
 Release 1.0
 ===========
 
+.. note::
+
+   This document describes a pre-release version of IPython.
+
 IPython 1.0 requires Python ≥ 2.6.5 or ≥ 3.2.1.
 It does not support Python 3.0, 3.1, or 2.5.