Align PR template with PyMC

Author: michaelosthege

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,14 +1,25 @@
+<!-- !! Thank your for opening a PR !! -->
 
-**Thank you for opening a PR!**
+### Motivation for these changes
+...
 
-Here are a few important guidelines and requirements to check before your PR can be merged:
-+ [ ] There is an informative high-level description of the changes.
-+ [ ] The description and/or commit message(s) references the relevant GitHub issue(s).
-+ [ ] [`pre-commit`](https://pre-commit.com/#installation) is installed and [set up](https://pre-commit.com/#3-install-the-git-hook-scripts).
-+ [ ] The commit messages follow [these guidelines](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
-+ [ ] The commits correspond to [_relevant logical changes_](https://wiki.openstack.org/wiki/GitCommitMessages#Structural_split_of_changes), and there are **no commits that fix changes introduced by other commits in the same branch/BR**.
-+ [ ] There are tests covering the changes introduced in the PR.
+### Implementation details
+...
 
-Don't worry, your PR doesn't need to be in perfect order to submit it.  As development progresses and/or reviewers request changes, you can always [rewrite the history](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History#_rewriting_history) of your feature/PR branches.
 
-If your PR is an ongoing effort and you would like to involve us in the process, simply make it a [draft PR](https://docs.github.com/en/free-pro-team@latest/github/collaborating-with-issues-and-pull-requests/about-pull-requests#draft-pull-requests).
+### Checklist
++ [ ] Explain motivation and implementation 👆
++ [ ] Make sure that [the pre-commit linting/style checks pass](https://docs.pymc.io/en/latest/contributing/python_style.html).
++ [ ] Link relevant issues, preferably in [nice commit messages](https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html).
++ [ ] The commits correspond to [_relevant logical changes_](https://wiki.openstack.org/wiki/GitCommitMessages#Structural_split_of_changes). Note that if they don't, we will [rewrite/rebase/squash the git history](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History#_rewriting_history) before merging.
++ [ ] Are the changes covered by tests and docstrings?
++ [ ] Fill out the short summary sections 👇
+
+## Major / Breaking Changes
+- ...
+
+## Bugfixes / New features
+- ...
+
+## Docs / Maintenance
+- ...