Add pull request template

.github/pull_request_template.md

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+<!-- Thank you for working on Jamulus and opening a Pull Request! Please fill the following to make the review process straight foward -->
+
+**Short description of changes**
+
+<!-- A short description of your changes which might go into the change log -->
+
+**Context: Fixes an issue?**
+
+<!-- If this fixes an issue, please write Fixes: <issue number here>; if not, please give your PR a context. -->
+
+**Does this change need documentation? What needs to be documented and how?**
+
+<!-- Most new features should be documented on the website: https://github.com/jamulussoftware/jamuluswebsite/ If you have a proposal what to document, feel free to open a draft PR on the website repo -->
+
+**Status of this Pull Request**
+<!-- This might be edited by maintainers. -->
+<!-- Proof of concept (not to be merged soon); Working implementation; ... -->
+
+**What is missing until this pull request can be merged?**
+<!-- Does it still need more testing; ... -->
+
+## Checklist
+<!-- Please tick the check boxes when done by replacing the space by an x, e.g. [x]. -->
+- [ ] I've verified that this Pull Request follows the [general code principles](https://github.com/jamulussoftware/jamulus/blob/master/CONTRIBUTING.md#jamulus-projectsource-code-general-principles)
+- [ ] I tested my code and it does what I want
+- [ ] My code follows the [style guide](https://github.com/jamulussoftware/jamulus/blob/master/CONTRIBUTING.md#source-code-consistency) <!-- You can also check if your code passes clang-format -->
+- [ ] I waited some time after this Pull Request was opened and all GitHub checks completed without errors. <!-- GitHub doesn't run these checks for new contributors automatically. -->
+- [ ] I've filled all the content above