Extract section from changelog

The automatic release notes did not include the tag annotation. Extract
notes from the changelog instead.

.github/workflows/release.yaml

@@ -1,7 +1,4 @@
 # Cut a release whenever a new tag is pushed to the repo.
-# You should use an annotated tag, like `git tag -a v1.2.3 --cleanup=whitespace`
-# and put the release notes into the commit message for the tag.
-# The `--cleanup` flag avoids markdown headings being stripped as comments.
 name: Release
 
 on:
@@ -24,8 +21,19 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v3
+      - name: Get version number
+        id: get-version
+        run: set -x; echo "version=${GITHUB_REF_NAME#v}" >> $GITHUB_OUTPUT
+      - name: Read section from CHANGELOG.md
+        id: extract-changelog
+        uses: sean0x42/markdown-extract@v2
+        with:
+         file: CHANGELOG.md
+         pattern: ${{ steps.get-version.outputs.version }}
       - name: Prepare release notes and artifacts
-        run: .github/workflows/release_prep.sh ${{ env.GITHUB_REF_NAME }} > release_notes.txt
+        run: |
+          .github/workflows/release_prep.sh ${{ env.GITHUB_REF_NAME }} > release_notes.txt
+          printf '${{ steps.extract-changelog.outputs.markdown }}' >> release_notes.txt
       - name: Release
         uses: softprops/action-gh-release@v1
         with: