README for documentation helpers

doc/sphinx/helpers/README.md

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+# Documentation helpers for developers
+
+This directory contains helpers for working with documentation.
+
+## [milestone_issues_and_prs.py](milestone_issues_and_prs.py)
+
+The Sphinx-based changelog consists of the reduction of issues and PRs to the repository between each version tag.
+This script generates a Sphinx fragment containing the raw issue and PR titles, numbers, and authors that have been assigned to a GitHub milestone.
+These should then be summarized and combined into more useful changelog entries.
+
+In order to run the script (`python milestone_issues_and_prs.py > milestone_issues_and_prs.rst`),
+
+- export the environment variable `GITHUB_API_TOKEN` to contain a Personal Access Token that has ~read privileges
+- change the name of the milestone in [milestone_issues_and_prs.graphql](milestone_issues_and_prs.graphql) to match the upcoming release milestone