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

native_client_sdk/doc_generated/reference/nacl-manifest-format.html

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 <section id="native-client-manifest-nmf-format">
 <h1 id="native-client-manifest-nmf-format">Native Client Manifest (nmf) Format</h1>
 <div class="contents local" id="contents" style="display: none">
 <ul class="small-gap">
 <li><a class="reference internal" href="#overview" id="id1">Overview</a></li>
 <li><a class="reference internal" href="#field-summary" id="id2">Field summary</a></li>
 <li><p class="first"><a class="reference internal" href="#field-details" id="id3">Field details</a></p>
 <ul class="small-gap">
@@ skipping 59 matching lines @@
         // url is required
         &quot;url&quot;: &quot;url_to_my_bitcode_bc&quot;,
 
         // optlevel is optional
         &quot;optlevel&quot;: 0
       }
     }
   }
 }
 </pre>
-<p>Portable Native Client applications can also specify an <code>optlevel</code> field.
+<p id="pnacl-nmf-optlevels">Portable Native Client applications can also specify an <code>optlevel</code> field.
 The <code>optlevel</code> field is an optimization level <em>hint</em>, which is a number
 (zero and higher). Higher numbers indicate more optimization effort.
 Setting a higher optimization level will improve the application&#8217;s
 performance, but it will also slow down the first load experience.
 The default is <code>optlevel</code> is currently <code>2</code>, and values higher
 than 2 are no different than 2. If compute speed is not as important
 as first load speed, an application could specify an <code>optlevel</code>
 of <code>0</code>. Note that <code>optlevel</code> is only a <em>hint</em>. In the future, the
 Portable Native Client translator and runtime may <em>automatically</em> choose
 an <code>optlevel</code> to best balance load time and application performance.</p>
@@ skipping 136 matching lines @@
 representing the contents of the file. Specifying the <code>nmf</code> contents
 inline with a data URI can help reduce the amount of network traffic
 required to load the Native Client application.</p>
 <h3 id="url-resolution"><span id="nmf-url-resolution"></span>URL resolution</h3>
 <p>All URLs contained in a manifest are resolved relative to the URL of
 the manifest. If the manifest was specified as a data URI, the URLs must
 all be absolute.</p>
 </section>
 
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