a PR template explaining the presence of a bot

Author: Krastanov

.github/pull_request_template.md

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+Please make sure to leave this PR editable by maintainers of this repo.
+
+Please make sure you have consulted [CONTRIBUTING](https://github.com/JuliaGraphs/Graphs.jl/blob/master/CONTRIBUTING.md)
+
+The linter bot might add new commits to your PR (e.g. to take care of formatting issues). Feel free to overwrite these commits.