MNT: add release message for rever

Author: st3107

rever.xsh

@@ -8,3 +8,7 @@ $ACTIVITIES = [
 $PUSH_TAG_REMOTE = '[email protected]:diffpy/diffpy.structure.git'  # Repo to push tags to
 $GITHUB_ORG = 'diffpy'  # Github org for Github releases and conda-forge
 $GITHUB_REPO = 'diffpy.structure'  # Github repo for Github releases  and conda-forge
+$GHRELEASE_PREPEND = """See [CHANGELOG.md](CHANGELOG.md) for detailed release notes.
+
+The release is also available at [PyPI](https://pypi.org/project/diffpy.structure/) and [Conda](https://anaconda.org/conda-forge/diffpy.structure).
+"""  # release message