Add NaCl SDK release note about LLVM 3.6 and debug metadata changes.

native_client_sdk/src/doc/sdk/release-notes.rst

 .. _sdk-release-notes:
 
 #############
 Release Notes
 #############
 
 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.
 
 
 Chrome/Pepper 43 (03 April 2015)
 ================================
 
 PNaCl
 -----
 
 * The C11/C++11 ``acquire``, ``release``, and ``acq_rel`` memory orders are now
   generated by default. The in-browser Chrome 42 translator supports them, the
   SDK can therefore generate them.
 * Fix a `code generation bug on ARM`_ when dealing with 16-bit load/store and
   ``bswap`` which led to a NaCl validation failure.
+* PNaCl is now based on LLVM 3.6. If you are using GDB to debug PNaCl
+  :ref:`BC files with debug metadata in the browser<debugging_pnacl_pexes>`,
+  remember that the SDK version must match the in-browser translator version.

[binji, 2015/04/01 16:59:00]

how can the developer know that the SDK version matches the translator version?

[jvoung (off chromium), 2015/04/01 17:22:56]

Reworded to say use SDK version X for chrome version X.

Technically they could run "pnacl-clang --version" to get a NaCl git hash, and
then go to chrome://nacl to also get a NaCl git hash, but the compatibility
could be a bit fuzzy (especially on canary/dev) where sometimes the SDK gets
ahead of the chrome translator (and that is okay).

+  Bitcode debug metadata changed from LLVM 3.5 to 3.6. Otherwise, it is still

[binji, 2015/04/01 16:59:00]

"Otherwise" sounds weird here. Otherwise what?

[jvoung (off chromium), 2015/04/01 17:22:56]

Reworded -- is this better?

+  possible to work around this version-match issue by translating pexes to
+  nexes using the :ref:`offline translator in the same SDK
+  <debugging_pexes_via_nexes>` instead of using the translator in
+  the browser.
 
 .. _`code generation bug on ARM`: https://code.google.com/p/chromium/issues/detail?id=460432
 
 Chrome/Pepper 42 (20 February 2015)
 ===================================
 
 SDK
 ---
 
 * The SDK now contains experimental versions of ``i686-nacl-clang``,
@@ skipping 680 matching lines @@
 
 PPAPI
 -----
 
 * The ``CompletionCallbackFactory`` class template now takes a thread traits
   class as its second parameter. For details see the `CompletionCallbackFactory
   class template reference
   </native-client/pepper_stable/cpp/classpp_1_1_completion_callback_factory#details>`_.
 
 .. TODO: Port release notes for older releases