add release template.

Author: janfb

.github/PULL_REQUEST_TEMPLATE.md renamed to .github/PULL_REQUEST_TEMPLATE/feature_or_bugfix.md

@@ -0,0 +1,39 @@
+# Preparing a release
+
+## Decide what will be the upcoming version number
+
+- `sbi` currently uses the [Semver 2.0.0](https://semver.org/) convention.
+- Edit the version number in the tuple at `sbi/sbi/__version__.py`.
+
+## Collect a list of relevant changes
+
+- [ ] Edit `changelog.md`, add a new version number header and report changes below it.
+- [ ] Use one line per change, include links to the pull requests that implemented each of
+  the changes.
+- [ ] **Credit contributors**!
+- [ ] If there are new package dependencies or updated version constraints for the existing
+  dependencies, add/modify the corresponding entries in `pyproject.toml`.
+
+## Run tests locally and make sure they pass
+
+- Run the **full test suite, including slow tests.**
+  - [ ] slow tests are passing
+  - [ ] GPU tests are passing
+
+## Upload to pypi
+
+The upload to `pypi` will happen automatically once a release is made
+via GitHub.
+
+To do so, **after merging this PR**, you need to
+
+- [ ] copy the new content you added to `changelog.md` to the clipboard
+- [ ] draft a new release here: https://github.com/sbi-dev/sbi/releases
+- [ ] create a new tag using the `vX.XX.X` scheme
+- [ ] paste the content of the `changelog` you copied above and edit it where needed
+- [ ] select "pre-release" if needed (default no) or "latest release" (default yes)
+- [ ] select "create a discussion" if there are breaking or important changes and users
+  should have a platform to discuss issues and questions.
+- [ ] "publish" or "draft" the release.
+
+Once the release is *published* via Github, the upload to PyPi will be triggered.