Add issue and pull request templates

docs/pull_request_template.md

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+<!--
+Before submitting a pull request you are strongly recommended to read the
+CONTRIBUTING.md file which gives some information on how to prepare a change.
+
+For small changes you don't have to read the document end to end, but should at
+least look at the sections on how to ensure your code and commits are formatted
+according to the style requirements.
+-->
+
+## Description
+
+<!--
+Describe here what your pull request changes and why it should happen. For small
+changes which are obvious this can just be a line or two - even the commit
+message is sometimes enough.
+-->