[NaClDocs] Fill in the landing page for the SDK section of the docs.

native_client_sdk/src/doc/sdk/index.rst

 .. _sdk-index:
 
 #################
 Native Client SDK
 #################
 
-This is the index of the ``sdk`` directory.
+To build Native Client modules, you must download and install the Native
+Client Software Development Kit (SDK).
 
+The SDK includes the following:
+
+support for multiple Pepper versions
+  The SDK contains **bundles** that let you compile Native Client modules
+  using different versions of the
+  `Pepper API </overview.html#link_pepper>`_
+  (e.g., Pepper 30 or Pepper Canary). Review the
+  :doc:`Release Notes <release-notes>` for a description of the new features
+  included in each Pepper version to help you decide which bundle to
+  use to develop your application. In general, Native Client modules
+  compiled using a particular Pepper version will work in
+  corresponding versions of Chrome and higher. For example, a module
+  compiled using the Pepper 30 platform will work in Chrome 30 and
+  higher.
+
+update utility
+  The ``naclsdk`` utility (``naclsdk.bat`` on Windows) lets you download new
+  bundles that are available, as well as new versions of existing bundles.
+
+toolchains
+  Each platform includes three toolchains---one for Portable Native Client
+  (PNaCl) applications, one for compiling architecture-specific Native
+  Client applications with newlib, and a third native NaCl toolchain for
+  compiling with glibc. Newlib and glibc are two different implementations
+  of the C standard library. All toolchains contain Native Client-compatible
+  versions of standard compilers, linkers, and other tools.
+  See :doc:`this document </nacl-and-pnacl>` to help you choose the
+  right toolchain.
+
+examples
+  Each example in the SDK includes C or C++ source files and header files
+  illustrating how to use NaCl and Pepper, along with a Makefile to build
+  the example using each of the toolchains.
+
+tools
+  The SDK includes a number of additional tools, which you can use for
+  tasks such as validating Native Client modules and running modules
+  from the command line.
+
+To download and install the SDK, follow the instructions on the
+:ref:`Download <download>` page.
+