github: add PR template

.github/PULL_REQUEST_TEMPLATE.md

+<!--
+The RIOT community cares a lot about code quality.
+Therefore, before describing what your contribution is about, we would like
+you to make sure that your modifications are compliant with the RIOT
+coding conventions, see https://github.com/RIOT-OS/RIOT/wiki/Coding-conventions.
+-->
+
+### Contribution description
+
+<!--
+Put here the description of your contribution:
+- describe which part(s) of RIOT is (are) involved
+- if it's a bug fix, describe the bug that it solves and how it is solved
+- you can also give more information to reviewers about how to test your changes
+-->
+
+
+### Issues/PRs references
+
+<!--
+Examples: Fixes #1234. See also #5678. Depends on PR #9876.
+
+Please use keywords (e.g., fixes, resolve) with the links to the issues you
+resolved, this way they will be automatically closed when your pull request
+is merged. See https://help.github.com/articles/closing-issues-using-keywords/.
+-->
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