Add release docs from github wiki

Signed-off-by: Julius Härtl <jus@bitgrid.net>

docs/Release.md

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+Releasing a new version works quite easy with [krankerl](https://github.com/ChristophWurst/krankerl) and [github-release](https://github.com/aktau/github-release) installed:
+
+1. Run krankerl to build the package
+
+```
+krankerl package
+```
+
+2. Tag the release on GitHub
+
+```
+# For a prerelease
+github-release release -u nextcloud -r deck -t v0.3.1 -p
+
+# For a regular release
+github-release release -u nextcloud -r deck -t v0.3.1
+```
+
+3. Upload the release package to GitHub
+
+```
+github-release upload -u nextcloud -r deck -t v0.3.1 -n deck.tar.gz -f build/artifacts/deck.tar.gz
+```
+
+4. Run krankerl to release the package to the app store (add `--nightly` for prerelease packages)
+
+```
+krankerl publish https://github.com/nextcloud/deck/releases/download/v0.3.1/deck.tar.gz  
+```
+
+## Release PR template
+
+```
+## Backports
+
+- [ ] ...
+
+## Translations
+
+- [ ] ...
+
+## Release
+
+- [ ] Set proper Nextcloud versions in info.xml
+- [ ] Update changelog
+- [ ] Build test release
+- [ ] Tested on 
+  - [ ] Nextcloud 13
+  - [ ] Nextcloud 14
+  - [ ] Nextcloud 15
+- [ ] Merge
+- [ ] Build final release
+- [ ] Publish release
+- [ ] Upload to the app store
+```