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

native_client_sdk/doc_generated/sdk/release-notes.html

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 <section id="release-notes">
 <span id="sdk-release-notes"></span><h1 id="release-notes"><span id="sdk-release-notes"></span>Release Notes</h1>
 <p>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.</p>
 <h2 id="chrome-pepper-43-03-april-2015">Chrome/Pepper 43 (03 April 2015)</h2>
 <h3 id="pnacl">PNaCl</h3>
 <ul class="small-gap">
 <li>The C11/C++11 <code>acquire</code>, <code>release</code>, and <code>acq_rel</code> memory orders are now
 generated by default. The in-browser Chrome 42 translator supports them, the
 SDK can therefore generate them.</li>
 <li>Fix a <a class="reference external" href="https://code.google.com/p/chromium/issues/detail?id=460432">code generation bug on ARM</a> when dealing with 16-bit load/store and
 <code>bswap</code> which led to a NaCl validation failure.</li>
 <li>PNaCl is now based on LLVM 3.6. If you are using GDB to debug PNaCl
 <a class="reference internal" href="/native-client/devguide/devcycle/debugging.html#debugging-pnacl-pexes"><em>BC files with debug metadata in the browser</em></a>,
 remember that debug info from SDK version <code>X</code> is only compatible with the
 PNaCl translator in chrome version <code>X</code>. The bitcode debug metadata format
 changed from LLVM 3.5 to 3.6. If you need to debug an app built with SDK
 version <code>X</code> running in Chrome version <code>Y</code> (with <code>X != Y</code>), it is still
 possible to do so. Simply translate the pexe to a nexe using the
 <a class="reference internal" href="/native-client/devguide/devcycle/debugging.html#debugging-pexes-via-nexes"><em>offline pnacl-translate tool from SDK version X</em></a> instead of using the translator in the
 browser (version <code>Y</code>).</li>
 <li>PNaCl&#8217;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.</li>
+<li>PNaCl&#8217;s experimental <a class="reference external" href="https://chromium.googlesource.com/native_client/pnacl-subzero/+/master/README.rst">Subzero translator</a> is available for x86-32 NaCl in
+Chrome version 43, behind a flag. To give it a try, run Chrome with the
+<code>--enable-pnacl-subzero</code> commandline flag, and use the <code>optlevel 0</code>
+<a class="reference internal" href="/native-client/reference/nacl-manifest-format.html#pnacl-nmf-optlevels"><em>NaCl manifest option</em></a>. Application startup time
+should be several times faster than the previous LLVM-based <code>optlevel 0</code>
+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 <a class="reference external" href="https://groups.google.com/forum/#!forum/native-client-dev">native-client-dev</a>. We are working on improvements and adding
+new targets.</li>
 </ul>
 <h3 id="pepper">Pepper</h3>
 <ul class="small-gap">
 <li>UDP Socket Multicast API in development preview (PPB_UDP_SOCKET 1.2).</li>
 <li>Hardware Video Encoder API in development preview (PPB_VIDEO_ENCODER 0.1).</li>
 </ul>
 <h2 id="chrome-pepper-42-20-february-2015">Chrome/Pepper 42 (20 February 2015)</h2>
 <h3 id="sdk">SDK</h3>
 <ul class="small-gap">
 <li>The SDK now contains experimental versions of <code>i686-nacl-clang</code>,
@@ skipping 588 matching lines @@
 </ul>
 <h3 id="id30">PPAPI</h3>
 <ul class="small-gap">
 <li>The <code>CompletionCallbackFactory</code> class template now takes a thread traits
 class as its second parameter. For details see the <a class="reference external" href="/native-client/pepper_stable/cpp/classpp_1_1_completion_callback_factory#details">CompletionCallbackFactory
 class template reference</a>.</li>
 </ul>
 </section>
 
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