Add review checklist to new PRs (#1609)

Co-authored-by: Hendrik Ranocha <[email protected]>

.github/review-checklist.md

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+### Review checklist
+
+This checklist is meant to assist creators of PRs (to let them know what reviewers will typically look for) and reviewers (to guide them in a structured review process). Items do not need to be checked explicitly for a PR to be eligible for merging.
+
+#### Purpose and scope
+- [ ] The PR has a single goal that is clear from the PR title and/or description.
+- [ ] All code changes represent a single set of modifications that logically belong together.
+- [ ] No more than 500 lines of code are changed or there is no obvious way to split the PR into multiple PRs.
+
+#### Code quality
+- [ ] The code can be understood easily.
+- [ ] Newly introduced names for variables etc. are self-descriptive and consistent with existing naming conventions.
+- [ ] There are no redundancies that can be removed by simple modularization/refactoring.
+- [ ] There are no leftover debug statements or commented code sections.
+- [ ] The code adheres to our [conventions](https://trixi-framework.github.io/Trixi.jl/stable/conventions/) and [style guide](https://trixi-framework.github.io/Trixi.jl/stable/styleguide/), and to the [Julia guidelines](https://docs.julialang.org/en/v1/manual/style-guide/).
+
+#### Documentation
+- [ ] New functions and types are documented with a docstring or top-level comment.
+- [ ] Relevant publications are referenced in docstrings (see [example](https://github.com/trixi-framework/Trixi.jl/blob/7f83a1a938eecd9b841efe215a6e482e67cfdcc1/src/equations/compressible_euler_2d.jl#L601-L615) for formatting).
+- [ ] Inline comments are used to document longer or unusual code sections.
+- [ ] Comments describe intent ("why?") and not just functionality ("what?").
+- [ ] If the PR introduces a significant change or new feature, it is documented in `NEWS.md`.
+
+#### Testing
+- [ ] The PR passes all tests.
+- [ ] New or modified lines of code are covered by tests.
+- [ ] New or modified tests run in less then 10 seconds.
+
+#### Performance
+- [ ] There are no type instabilities or memory allocations in performance-critical parts.
+- [ ] If the PR intent is to improve performance, before/after [time measurements](https://trixi-framework.github.io/Trixi.jl/stable/performance/#Manual-benchmarking) are posted in the PR.
+
+#### Verification
+- [ ] The correctness of the code was verified using appropriate tests.
+- [ ] If new equations/methods are added, a convergence test has been run and the results
+  are posted in the PR.
+
+*Created with :heart: by the Trixi.jl community.*

.github/workflows/ReviewChecklist.yml

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+name: Add review checklist
+
+on:
+  pull_request_target:
+    types: [opened]
+
+permissions:
+  pull-requests: write
+
+jobs:
+  add-review-checklist:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check out repository
+        uses: actions/checkout@v3
+      - name: Add review checklist
+        uses: trixi-framework/add-pr-review-checklist@v1
+        with:
+          file: '.github/review-checklist.md'
+          no-checklist-keyword: '[no checklist]'