Create pull_request_template.md

.github/pull_request_template.md

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+### Description of the change
+
+*Please describe what has changed and why. If it fixes an open issue, mention it here.*
+
+### Checklist for contributor
+
+* [ ] if you want to be mentioned in the [AUTHORS](https://github.com/progsource/maddy/blob/master/AUTHORS) file, you added yourself
+* [ ] added entry to [CHANGELOG.md](https://github.com/progsource/maddy/blob/master/CHANGELOG.md) in "Upcoming"
+* [ ] if any Markdown definition changed, you updated the [definition docs](https://github.com/progsource/maddy/blob/master/docs/definitions.md)
+* [ ] your C++ code change is accommodated with a unit/integration test where it makes sense
+* [ ] your code meets the code format style (clang-format) of the project (`tools/format.py`)