templates: add release checklist

Imported verbatim from wiki HowToRelease.

Author: bgilbert

.github/ISSUE_TEMPLATE/release.md

@@ -0,0 +1,16 @@
+# OpenSlide Python release process
+
+- [ ] Update `CHANGELOG.txt` and version in `openslide/_version.py`
+- [ ] Create and push signed tag
+- [ ] [Launch AppVeyor build](https://ci.appveyor.com/project/OpenSlide/openslide-python)
+- [ ] `git clean -dxf && mkdir dist`
+- [ ] Download wheels from each build job into `dist` directory
+- [ ] `python setup.py register sdist`
+- [ ] `twine upload dist/*`
+- [ ] Recompress tarball with `xz`
+- [ ] Attach release notes to [GitHub release](https://github.com/openslide/openslide-python/releases); upload tarballs and wheels
+- [ ] `python setup.py build_sphinx` and copy `build/sphinx/html/` to website `api/python/`
+- [ ] Update website: `_data/releases.yaml`, `_includes/news.markdown`
+- [ ] Send mail to -announce and -users
+- [ ] Update Fedora package
+- [ ] Update MacPorts package