templates: add release checklist

Imported verbatim from wiki HowToRelease.

Author: bgilbert

.github/ISSUE_TEMPLATE/release.md

@@ -0,0 +1,10 @@
+# OpenSlide Java release process
+
+- [ ] Run test build and `make distcheck`
+- [ ] Update `CHANGELOG.txt` and version in `configure.ac`
+- [ ] Create and push signed tag
+- [ ] `git clean -dxf && autoreconf -i && ./configure && make distcheck`
+- [ ] Attach release notes to [GitHub release](https://github.com/openslide/openslide-java/releases), set pre-release flag, and upload tarballs
+- [ ] Update openslide-winbuild
+- [ ] Update website: `_data/releases.yaml`, `_includes/news.markdown`
+- [ ] Send mail to -announce and -users