[DOCS] Add supported OSs and platforms to the SDK download page.

native_client_sdk/src/doc/sdk/download.rst

 .. _download:
 
 Download the Native Client SDK
 ==============================
 
 This page provides an overview of the Native Client SDK, and instructions for
 downloading and installing the SDK.
 
 .. raw:: html
   
@@ skipping 55 matching lines @@
   several other useful tools, is to install `Xcode Developer Tools 
   <https://developer.apple.com/technologies/tools/>`_. After installing Xcode,
   go to the XCode menu, open the Preferences dialog box then select Downloads
   and Components. Verify that Command Line Tools are installed. If you'd rather
   not install Xcode, you can download and build an `open source version 
   <http://mac.softpedia.com/dyn-postdownload.php?p=44632&t=4&i=1>`_ of ``make``.
   To build the program you may also need to download and install `gcc 
   <https://github.com/kennethreitz/osx-gcc-installer>`_.
 * On Windows, the Native Client SDK includes a copy of GNU Make.
 
+.. _platforms:
+
+Platforms
+---------
+
+Native Client supports several operating systems, including Windows, Linux, OSX,
+and ChromeOS. It supports several architectures including on x86-32, x86-64,
+ARM, and MIPS.
+
 .. _versioning:
 
-Versioning
-----------
+Versions
+--------
 
 Chrome is released on a six week cycle, and developer versions of Chrome are
 pushed to the public beta channel three weeks before each release. As with any
 software, each release of Chrome may include changes to Native Client and the
 Pepper interfaces that may require modification to existing applications.
 However, modules compiled for one version of Pepper/Chrome should work with
 subsequent versions of Pepper/Chrome. The SDK includes multiple versions of the
 Pepper APIs to help developers make adjustments to API changes and take
 advantage of new features: `stable </native-client/pepper_stable>`_, `beta
 </native-client/pepper_beta>`_ and `dev </native-client/pepper_dev>`_.
@@ skipping 171 matching lines @@
 Next steps
 ----------
 
 * Browse the `Release Notes <release-notes>`_ for important
   information about the SDK and new bundles.
 * If you're just starting with Native Client, we recommend reading the 
   `Technical Overview <../overview>`_ and walking through the
   `Getting Started Tutorial <devguide/tutorial/tutorial-part1>`_.
 * If you'd rather dive in, see
   `Building Native Client Modules <devguide/devcycle/building>`_.