docs: Update instructions for requesting backports to the release branches

[lattner]

This patch LGTM, I think we should also expand the github page to include other hints and tips! :-)

llvm/docs/GettingInvolved.rst

.. toctree::
    DeveloperPolicy
    CodeReview
    SupportPolicy
    SphinxQuickstartTemplate
    Phabricator
    HowToSubmitABug
    BugLifeCycle
    CodingStandards
+   GitHub
    GitBisecting
    GitRepositoryPolicy
 
 :doc:`Contributing`
    An overview on how to contribute to LLVM.
 
 :doc:`DeveloperPolicy`
    The LLVM project's policy towards developers and their contributions.

 
 :doc:`BugLifeCycle`
    Describes how bugs are reported, triaged and closed.
 
 :doc:`CodingStandards`
   Details the LLVM coding standards and provides useful information on writing
   efficient C++ code.
 
+:doc:`GitHub`
+  Describes how to use the llvm-project repository on GitHub.
+
 :doc:`GitBisecting`
   Describes how to use ``git bisect`` on LLVM's repository.
 
 :doc:`GitRepositoryPolicy`
    Collection of policies around the git repositories.
 
 .. _development-process:
 

llvm/docs/GitHub.rst

+======================
+LLVM GitHub User Guide
+======================
+
+Introduction
+============
+The LLVM Project uses `GitHub <https://github.com/>`_ for
+`Source Code <https://github.com/llvm/llvm-project>`_,
+`Releases <https://github.com/llvm/llvm-project/releases>`_, and
+`Issue Tracking <https://github.com/llvm/llvm-project/issues>`_.
+
+This page describes how the LLVM Project users and developers can
+participate in the project using GitHub.
+
+Releases
+========
+
+Backporting Fixes to the Release Branches
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+You can use special comments on issues to make backport requests for the
+release branches.  This is done by making a comment containing one of the
+following commands on any issue that has been added to one of the "X.Y.Z Release"
+milestones.
+
+::
+
+  /cherry-pick <commit> <commit> <...>
+
+This command takes one or more git commit hashes as arguments and will attempt
+to cherry-pick the commit(s) to the release branch.  If the commit(s) fail to
+apply cleanly, then a comment with a link to the failing job will be added to
+the issue.  If the commit(s) do apply cleanly, then a pull request will
+be created with the specified commits.
+
+::
+
+  /branch <owner>/<repo>/<branch>
+
+This command will create a pull request against the latest release branch using
+the <branch> from the <owner>/<repo> repository.  <branch> cannot contain any
+forward slash '/' characters.

llvm/docs/HowToReleaseLLVM.rst

 To help manage all the bugs reported and which ones are blockers or not, a new
 "[meta]" bug should be created and all regressions *blocking* that Meta. Once
 all blockers are done, the Meta can be closed.
 
 If a bug can't be reproduced, or stops being a blocker, it should be removed
 from the Meta and its priority decreased to *normal*. Debugging can continue,
 but on trunk.
 
-Merge Requests
---------------
-
-You can use any of the following methods to request that a revision from trunk
-be merged into a release branch:
-
-#. Use the ``utils/release/merge-request.sh`` script which will automatically
-   file a bug_ requesting that the patch be merged. e.g. To request revision
-   12345 be merged into the branch for the 5.0.1 release:
-   ``llvm.src/utils/release/merge-request.sh -stable-version 5.0 -r 12345 -user [e-mail]``
-
-#. Manually file a bug_ with the subject: "Merge r12345 into the X.Y branch",
-   enter the commit(s) that you want merged in the "Fixed by Commit(s)" and mark
-   it as a blocker of the current release bug.  Release bugs are given aliases
-   in the form of release-x.y.z, so to mark a bug as a blocker for the 5.0.1
-   release, just enter release-5.0.1 in the "Blocks" field.
-
-#. Reply to the commit email on llvm-commits for the revision to merge and cc
-   the release manager.
+Backport Requests
+-----------------
 
-.. _bug: https://bugs.llvm.org/
+Instructions for requesting a backport to a stable branch can be found :doc:`here <GitHub>`.
 
 Release Patch Rules
 -------------------
 
 Below are the rules regarding patching the release branch:
 
 #. Patches applied to the release branch may only be applied by the release
    manager, the official release testers or the code owners with approval from