Release template added

.gitlab/issue_templates/Release.md

+**source code and binary**
+  * [ ] create branch YEAR.N
+  * [ ] create issue "Release version YEAR.N" and merge request
+  * [ ] update version string in Doxyfile
+  * [ ] update the version string in CMakeLists.txt and commit
+  * [ ] wait for approval of MR and merge
+  * [ ] tag version YEAR.N.0
+  * [ ] upload source tar-ball to `/afs/psi.ch/project/amas/webhosting/Downloads/OPAL/src`
+  * [ ] compile new binary for Linux
+  * [ ] upload Linux binary package to `/afs/psi.ch/project/amas/webhosting/Downloads/OPAL/package/`
+  * [ ] compile new binary for macOS
+  * [ ] upload macOS binary package to `/afs/psi.ch/project/amas/webhosting/Downloads/OPAL/package/`
+
+**manual/documentation**
+  * [ ] setup a new branch for the new version of the manual
+  * [ ] fix version, branches and links in `Manual.attributes`
+  * [ ] clone repository into `/afs/psi.ch/project/amas/webhosting/opal/Documentation/x.y` and checkout new branch (`git clone https://gitlab.psi.ch/OPAL/documentation/manual.git`)
+  * [ ] add links to the binaries in the wiki
+  * [ ] update https://gitlab.psi.ch/OPAL/src/wikis/For-Developers/Compile-OPAL
+  * [ ] compile the change log/release notes and publish it in the wiki: https://gitlab.psi.ch/OPAL/src/wikis/ReleaseNotes
+  * [ ] review the file `src/addToDoxygenMainPage.h`
+  * [ ] build Doxygen documentation
+  * [ ] update https://gitlab.psi.ch/OPAL/src/wikis/home
+  * [ ] update https://gitlab.psi.ch/OPAL/src/wikis/regression-tests
+
+**tracker**
+  * [ ] new milestone for `OPAL x.(y+1)`
+  * [ ] update labels and milestones in issues
+
+**regression-tests**
+
+  * [ ] create new branch x.y
+  * [ ] setup the regression-tests to run the new version on opalrunner.psi.ch
+
+**varia**
+  * [ ] PSI module
+  * [ ] write e-mail to mailing list