Add PR-Template

Add basic checklists for a simple PR-template

.github/PULL_REQUEST_TEMPLATE/simple.md

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+# What does this PR do?
+
+* Describe what the PR does
+* Reference issue/pr/discussion numbers and use the appropriate github-keywords
+
+# Checklist before creating a non-draft PR
+
+- [ ] All tests are passing
+- [ ] Clippy has no complains
+- [ ] Code is `rustfmt` formatted
+- [ ] Applicable labels are chosen (Note: it is not necessary to replicate the labels from the related issues)
+- [ ] There are no other open [Pull Requests](https://github.com/ellmau/adf-obdd/pulls) for the same update/change.
+  - [ ] If there is a good reason to have another PR for the same update/change, it is well justified.
+
+# Checklist on Guidelines and Conventions
+
+- [ ] Commit messages follow our guidelines
+- [ ] Code is self-reviewed
+- [ ] Naming conventions are met
+- [ ] New features are tested
+  - [ ] `quickcheck` has been considered
+  - [ ] All variants are considered and checked
+- Clippy Compiler-exceptions
+  - [ ] Used in a sparse manner
+  - [ ] If used, a separate comment describes and justifies its use
+- [ ] `rustdoc` comments are self-reviewed and descriptive
+- Error handling
+  - [ ] Use of `panic!(...)` applications is justified on non-recoverable situations
+  - [ ] `expect(...)` is used over `unwrap()` (except obvious test-cases)
+- [ ] No unsafe code (exceptions need to be discussed specifically)