Create PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

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+### Description
+- Insert a clear and concise description of what the PR is about.
+- Optionally, make a reference to an existing [issue](https://github.com/understandable-machine-intelligence-lab/Quantus/issues).
+- Describe the expected improvements of Quantus that results from this PR. Why was this work completed?
+
+### Implemented changes
+- Insert a description of the changes implemented in the pull request.
+  - [ ] Change 1
+  - [ ] Change 2
+  - [ ] Change 3
+
+### Minimum acceptance criteria
+- Specify what is necessary for the PR to be merged with the main branch.
+- @mentions of the person that is apt to review these changes e.g., @annahedstroem