[clang-tools-extra] Add release notes documentation

[LegalizeAdulthood]

Summary

Add a file for Extra Clang Tools release notes, based on 3.9 release notes file for Clang.

[LegalizeAdulthood]

Sort check names in list

docs/clang-tidy/checks/list.rst

91

This check was missing from the list and caused a sphinx error.

[alexfh]

Looks good with one comment. Thank you, Richard!

docs/ReleaseNotes.rst

30

"please check out the main" seems to have been left here unintentionally.

[alexfh]

Looks good with one comment. Thank you, Richard!

docs/clang-tidy/checks/list.rst

91

Please remove ".rst".

[LegalizeAdulthood]

Update from review comments.

I do not have commit access.

Patch by Richard Thomson

docs/ReleaseNotes.rst

+=================================================
+Extra Clang Tools 3.9 (In-Progress) Release Notes
+=================================================
+
+.. contents::
+   :local:
+   :depth: 2
+
+Written by the `LLVM Team <http://llvm.org/>`_
+
+.. warning::
+
+   These are in-progress notes for the upcoming Clang 3.9 release. You may
+   prefer the `Clang 3.8 Release Notes
+   <http://llvm.org/releases/3.8.0/tools/clang/docs/ReleaseNotes.html>`_.
+
+Introduction
+============
+
+This document contains the release notes for the Extra Clang Tools, part of the
+Clang release 3.9.  Here we describe the status of the Extra Clang Tools in some
+detail, including major improvements from the previous release and new feature
+work. For the general Clang release notes, see `the Clang documentation
+<http://llvm.org/releases/3.8.0/tools/clang/docs/ReleaseNotes.html>`_.  All LLVM
+releases may be downloaded from the `LLVM releases web
+site <http://llvm.org/releases/>`_.
+
+For more information about Clang or LLVM, including information about
+the latest release, please see the `Clang Web Site <http://clang.llvm.org>`_ or
+the `LLVM Web Site <http://llvm.org>`_.
+
+Note that if you are reading this file from a Subversion checkout or the
+main Clang web page, this document applies to the *next* release, not
+the current one. To see the release notes for a specific release, please
+see the `releases page <http://llvm.org/releases/>`_.
+
+What's New in Extra Clang Tools 3.9?
+====================================
+
+Some of the major new features and improvements to Extra Clang Tools are listed
+here. Generic improvements to Extra Clang Tools as a whole or to its underlying
+infrastructure are described first, followed by tool-specific sections.
+
+Major New Features
+------------------
+
+- Feature1...
+
+Improvements to ``clang-query``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The improvements are...
+
+Improvements to ``clang-rename``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The improvements are...
+
+Improvements to ``clang-tidy``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+``clang-tidy``'s checks are constantly being improved to catch more issues,
+explain them more clearly, and provide more accurate fix-its for the issues
+identified.  The improvements since the 3.8 release include:
+
+-  ...
+
+Improvements to ``modularize``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The improvements are...

docs/clang-tidy/checks/list.rst

.. toctree::
    modernize-shrink-to-fit
    modernize-use-auto
    modernize-use-default
    modernize-use-nullptr
    modernize-use-override
    performance-faster-string-find
    performance-for-range-copy
    performance-implicit-cast-in-loop
+   performance-unnecessary-copy-initialization
    readability-braces-around-statements
    readability-container-size-empty
    readability-else-after-return
    readability-function-size
    readability-identifier-naming
    readability-implicit-bool-cast
    readability-inconsistent-declaration-parameter-name
    readability-named-parameter
    readability-redundant-control-flow
    readability-redundant-smartptr-get
    readability-redundant-string-cstr
    readability-redundant-string-init
    readability-simplify-boolean-expr
    readability-uniqueptr-delete-release

docs/index.rst

 .. Extra Clang Tools documentation master file, created by
    sphinx-quickstart on Wed Feb 13 10:00:18 2013.
    You can adapt this file completely to your liking, but it should at least
    contain the root `toctree` directive.
 
 .. title:: Welcome to Extra Clang Tools's documentation!
 
 Introduction
 ============
 Welcome to the clang-tools-extra project which contains extra tools built using
 Clang's tooling API's.
 
+.. toctree::
+   :maxdepth: 1
+
+   ReleaseNotes
 
 Contents
 ========
 .. toctree::
    :maxdepth: 2
 
    clang-tidy/index
    modularize