Add merger role documentation

doc/merging.rst

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+How to merge a branch after approving the PR
+============================================
+
+Requirements
+--------------
+
+- A remote called *upstream* pointing at the central IC repository.
+
+- A remote configured for the author of the PR.
+
+- After the PR is approved, make sure the merge should be done (asking the authors if they didn't get in touch with you).
+
+
+Steps
+-------
+
+#. Fetch *upstream/master*.
+
+#. Reset or rebase your master branch to *upstream/master*.
+
+#. Fetch the branch of the approved PR.
+
+#. Create and check out a local branch on top of the PR branch.
+
+#. Make sure the branch of the approved PR is rebased onto *upstream/master*. If not:
+
+   * Rebase the branch onto *upstream/master*. If there are conflicts, ask the author to resolve them, unless they are obvious.
+
+   * Push to the branch of the PR and wait until the tests finish.
+
+#. Checkout your local *master*.
+
+#. Merge the PR branch into your local *master*, making sure that the merge commit conforms to our requirements. Here are the steps needed to make the merge happen:
+
+   * Disallow fast forward merging: we want an explicit merge commit for each PR.
+
+   * Edit the commit message as described above so that it has the following format:
+
+   <PR number>  <PR title>
+
+   <PR url>
+
+   [author: <author's id>]
+
+   <PR description>  (This is usually the whole first comment in the PR)
+
+   [reviewer: <approver's id>]
+
+   <Reviewer comment> (comment the reviewer left on GitHub)
+
+
+   * Ensure that Git records the reviewer of the PR as the author of the merge commit.
+
+#. Push the merge commit to *upstream/master*.
+
+#. Delete the local branch you created in step 4.