Document how to release pluggy

Author: nicoddemus

HOWTORELEASE.rst

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+Release Procedure
+-----------------
+
+#. Update the ``CHANGELOG.rst`` for the new release and commit.
+
+#. Open a PR named ``release-X.Y.Z`` targeting ``master``.
+
+#. All tests must pass and the PR must be approved by at least another maintainer.
+
+#. Publish to PyPI by pushing a tag::
+
+     git tag X.Y.Z release-X.Y.Z
+     git push [email protected]:pytest-dev/pluggy.git X.Y.Z
+
+   The tag will trigger a new build, which will deploy to PyPI.
+
+#. Make sure it is `available on PyPI <https://pypi.org/project/pluggy>`_.
+
+#. Merge ``release-X.Y.Z`` into ``master``, either manually or using GitHub's web interface.
+