Add migration to WebAssembly information to PNaCl / NaCl docs.

native_client_sdk/src/doc/migration/index.rst

+.. _migration:
+
+WebAssembly Migration Guide
+===========================
+
+(P)NaCl Deprecation Announcements
+---------------------------------
+
+Given the momentum of cross-browser WebAssembly support, we plan to focus our
+native code efforts on WebAssembly going forward and plan to remove support for
+PNaCl in Q1 2018 (except for Chrome Apps). We believe that the vibrant
+ecosystem around `WebAssembly <http://webassembly.org>`_
+makes it a better fit for new and existing high-performance
+web apps and that usage of PNaCl is sufficiently low to warrant deprecation.
+
+We also recently announced the deprecation Q1 2018 of
+`Chrome Apps
+<https://blog.chromium.org/2016/08/from-chrome-apps-to-web.html>`_
+outside of ChromeOS.
+
+
+Toolchain Migration
+-------------------
+
+For the majority of (P)NaCl uses cases we recommend transitioning
+from the NaCl SDK to `Emscripten
+<http://webassembly.org/getting-started/developers-guide/>`_.
+Migration is likely to be reasonably straightforward
+if your application is portable to Linux, uses
+`SDL <https://www.libsdl.org/>`_, or POSIX APIs.
+While direct support for NaCl / Pepper APIs in not available,
+we've attempted to list Web API equivalents.
+For more challenging porting cases, please reach out on
+native-client-discuss@googlegroups.com
+
+
+API Migration
+-------------
+
+We've outlined here the status of Web Platform substitutes for each
+of the APIs exposed to (P)NaCl.
+Additionally, the table lists the library or option in Emscripten
+that offers the closest substitute.
+
+We expect to add shared memory threads support to WebAssembly in 2017,
+as threads are crucial to matching (P)NaCl's most interesting use
+cases. Migration items which assume forthcoming threads support
+are marked below. If your application's flow control relies heavily on blocking
+APIs, you may also find threads support is required for convenient porting.
+
+While we've tried to be accurate in this table,
+there are no doubt errors or omissions.
+If you encounter one, please reach out to us on
+native-client-discuss@googlegroups.com
+
+.. contents::
+  :local:
+  :backlinks: none
+  :depth: 2
+
+PPAPI
+-----
+.. raw:: html
+  :file: public.html
+
+IRT
+---
+.. raw:: html
+  :file: public.html
+
+PPAPI (Apps)
+------------
+.. raw:: html
+  :file: apps.html