Add a clang release note about raising the minimum Windows version for the next major release

[gbedwell]

Summary

This is a clang release note to match the LLVM release note in D12367.

As we're planning on raising the minimum Windows version to Windows 7 for LLVM 3.8 as per this discussion (there have been no objections I'm aware of so far even after advertising the discussion in LLVM weekly):
http://lists.cs.uiuc.edu/pipermail/llvmdev/2015-July/087993.html
I'd like to add a release note to mention this fact.

[aaron.ballman]

LGTM for the release branch!

[hans]

Committed in r246090.

[gbedwell]

In D12368#233680, @hans wrote:

Committed in r246090.

Thanks!

cfe/branches/release_37/docs/ReleaseNotes.rst

 
 * readability-shrink-to-fit: replaces copy and swap tricks on shrinkable
   containers with the ``shrink_to_fit()`` method call.
 
 * readability-simplify-boolean-expr: looks for boolean expressions involving
   boolean constants and simplifies them to use the appropriate boolean
   expression directly (``if (x == true) ... -> if (x)``, etc.)
 
+Last release which will run on Windows XP and Windows Vista
+-----------------------------------------------------------
+
+This is expected to the be the last major release of Clang that will support
+running on Windows XP and Windows Vista.  For the next major release the
+minimum Windows version requirement will be Windows 7.
 
 Additional Information
 ======================
 
 A wide variety of additional information is available on the `Clang web
 page <http://clang.llvm.org/>`_. The web page contains versions of the
 API documentation which are up-to-date with the Subversion version of
 the source code. You can access versions of these documents specific to
 this release by going into the "``clang/docs/``" directory in the Clang
 tree.
 
 If you have any questions or comments about Clang, please feel free to
 contact us via the `mailing
 list <http://lists.llvm.org/mailman/listinfo/cfe-dev>`_.