Add PNaCl release note about how to try out Subzero / which flag to set.

native_client_sdk/src/doc/sdk/release-notes.rst

 .. _sdk-release-notes:
 
 #############
 Release Notes
 #############
 
 The dates in the following release notes denote when Chrome and the NaCl SDK
 reached canary status. The stable release is typically 6 weeks later.
 
 
@@ skipping 15 matching lines @@
   changed from LLVM 3.5 to 3.6. If you need to debug an app built with SDK
   version ``X`` running in Chrome version ``Y`` (with ``X != Y``), it is still
   possible to do so. Simply translate the pexe to a nexe using the
   :ref:`offline pnacl-translate tool from SDK version X
   <debugging_pexes_via_nexes>` instead of using the translator in the
   browser (version ``Y``).
 * PNaCl's support for use of libstdc++ 4.6 as the C++ standard library is
   deprecated and will be removed in the next release. PNaCl has used libc++
   (which is much more up-to-date, currently based on LLVM 3.6) as the default
   since Pepper 33.
+* PNaCl's experimental `Subzero translator`_ is available for x86-32 NaCl in
+  Chrome version 43, behind a flag. To give it a try, run Chrome with the
+  ``--enable-pnacl-subzero`` commandline flag, and use the ``optlevel 0``
+  :ref:`NaCl manifest option<pnacl_nmf_optlevels>`. Application startup time
+  should be several times faster than the previous LLVM-based ``optlevel 0``
+  mode, with similar code quality. Note that x86-32 NaCl requires a 32-bit
+  Chrome. On Windows, it also requires a 32-bit Windows OS, but 64-bit Linux
+  OSes can run x86-32 NaCl. If you try it out, please send us feedback
+  on `native-client-dev`_. We are working on improvements and adding
+  new targets.
 
 .. _`code generation bug on ARM`: https://code.google.com/p/chromium/issues/detail?id=460432
+.. _`Subzero translator`: https://chromium.googlesource.com/native_client/pnacl-subzero/+/master/README.rst
+.. _`native-client-dev`: https://groups.google.com/forum/#!forum/native-client-dev
 
 Pepper
 ------
 
 * UDP Socket Multicast API in development preview (PPB_UDP_SOCKET 1.2).
 * Hardware Video Encoder API in development preview (PPB_VIDEO_ENCODER 0.1).
 
 Chrome/Pepper 42 (20 February 2015)
 ===================================
 
@@ skipping 684 matching lines @@
 
 PPAPI
 -----
 
 * The ``CompletionCallbackFactory`` class template now takes a thread traits
   class as its second parameter. For details see the `CompletionCallbackFactory
   class template reference
   </native-client/pepper_stable/cpp/classpp_1_1_completion_callback_factory#details>`_.
 
 .. TODO: Port release notes for older releases