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

native_client_sdk/src/doc/reference/nacl-manifest-format.rst

 ###################################
 Native Client Manifest (nmf) Format
 ###################################
 
 .. contents::
   :local:
   :backlinks: none
   :depth: 2
 
 Overview
@@ skipping 59 matching lines @@
           // url is required
           "url": "url_to_my_bitcode_bc",
 
           // optlevel is optional
           "optlevel": 0
         }
       }
     }
   }
 
+.. _pnacl_nmf_optlevels:
+
 Portable Native Client applications can also specify an ``optlevel`` field.
 The ``optlevel`` field is an optimization level *hint*, which is a number
 (zero and higher). Higher numbers indicate more optimization effort.
 Setting a higher optimization level will improve the application's
 performance, but it will also slow down the first load experience.
 The default is ``optlevel`` is currently ``2``, 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 ``optlevel``
 of ``0``. Note that ``optlevel`` is only a *hint*. In the future, the
 Portable Native Client translator and runtime may *automatically* choose
@@ skipping 175 matching lines @@
 required to load the Native Client application.
 
 .. _nmf_url_resolution:
 
 URL resolution
 --------------
 
 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.