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

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

Index: native_client_sdk/src/doc/reference/nacl-manifest-format.rst
diff --git a/native_client_sdk/src/doc/reference/nacl-manifest-format.rst b/native_client_sdk/src/doc/reference/nacl-manifest-format.rst
index a85799875dda803800d0a85b49aed0d5a07c58de..2df2de1d162cff6b4d39aa12be0afd43d108f6ed 100644
--- a/native_client_sdk/src/doc/reference/nacl-manifest-format.rst
+++ b/native_client_sdk/src/doc/reference/nacl-manifest-format.rst
@@ -77,6 +77,8 @@ Example of a ``program`` for Portable Native Client:
     }
   }
 
+.. _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.