Created How to merge a PR (markdown)

How-to-merge-a-PR.md

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+1. Create you PR (ideally ask in an issue if that is a good idea what you want to do)
+2. Wait for a Maintainer review
+3. Do the requested changes
+4. Rebase with `dev` if its necessary:
+```
+git checkout dev
+git pull
+git checkout <the_branch_im_working_in>
+git rebase dev
+```