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

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

Index: native_client_sdk/doc_generated/reference/nacl-manifest-format.html
diff --git a/native_client_sdk/doc_generated/reference/nacl-manifest-format.html b/native_client_sdk/doc_generated/reference/nacl-manifest-format.html
index b796f0cc4c983b6c26463a9ef10907e59f63b402..69de516fca8a61f6de116b5b84c94f21fb3d2619 100644
--- a/native_client_sdk/doc_generated/reference/nacl-manifest-format.html
+++ b/native_client_sdk/doc_generated/reference/nacl-manifest-format.html
@@ -77,7 +77,7 @@ section for the rules on URL resolution.</p>
   }
 }
 </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