added note to docs about anvil format

docs/index.rst

@@ -12,6 +12,13 @@ See also the `Github Homepage`_ and the `Updates Blog`_
 
 .. _Updates Blog: http://overviewer.org/blog/
 
+.. warning::
+
+    This version of The Overviewer is being phased out. If you want to render
+    worlds for Minecraft 1.2 (the new "Anvil" format) then see the `Anvil <http://docs.overviewer.org/en/anvil/index.html>`_ docs.
+
+    Also see `this blog post <http://overviewer.org/blog/2012/03/01/anvil-support/>`_ on the topic.
+
 Introduction
 ============
 The Minecraft Overviewer is a command-line tool for rendering high-resolution