added release checklist template

CONTRIBUTING.md

@@ -162,6 +162,8 @@ All of this makes reviewing and bisecting for regressions easier.
 1. Squash merge only if the history is a mess or it makes more
 sense (eg. consecutive commits through the github website). 
 2. For release planning check the milestones and gemrb/docs/Release.txt.
+2.1. When starting the final approach, use `admin/github_release.checklist`
+as a template to track progress in a dedicated issue.
 3. Releases are usually made when larger pieces of work land or many smaller
 fixes have accrued.
 4. Versioning is semantic, but also ad-hoc. We want 1.0 to be the classic

admin/github_release.checklist

+### Release checklist
+
+This is a general release preparation checklist. See [Release.md](gemrb/docs/en/Release.md) for a longer writeup.
+
+Final development:
+- [ ] Fix or postpone issues in the [milestone](https://github.com/gemrb/gemrb/milestones)
+- [ ] Triage open pull requests
+- [ ] Check local stashes
+
+Beta testing:
+- [ ] Playthrough: Chateau Irenicus to Waukeen's promenade
+- [ ] Playthrough: TOB up to Saradush
+- [ ] Ideally a full game runthrough of one of the compleatable games
+- [ ] Retest character generation of remaining games
+
+Release:
+- [ ] Finalise choice of version number
+- [ ] Update both version numbers
+- [ ] Final `NEWS` update
+- [ ] Write docs:
+  - [ ] Forum announcements
+  - [ ] Release notes
+- [ ] `make dist`
+- [ ] Add a git tag and push it
+- [ ] Run `admin/guidoc_wikifier.sh` (no params needed) and upload the new docs
+- [ ] Promote the new tag to a release on GitHub
+- [ ] Update versions back to `-git` suffix
+- [ ] `admin/restart_news.sh`
+- [ ] SourceForge
+  - [ ] Upload source package to SourceForge
+    - [ ] Upload any release notes alongside
+  - [ ] Mark release's AppVeyor build as a Windows default on SourceForge
+    - [ ] (optional) regenerate android build
+  - [ ] Test downloads
+- [ ] Runtime tests of release packages
+- [ ] Announce as per Release.md