Add initial issue template and CONTRIBUTING.md

.github/ISSUE_TEMPLATE/blank_issue.md

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+---
+name: Blank Issue
+about: Create a blank issue.
+---

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug Report
+about: Create a bug report
+labels: bug
+---

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: true

CONTRIBUTING.md

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+# Contributing to Mosaic
+
+Thanks for considering to contribute to mosaic!
+
+Before contributing please read our [Code of Conduct](CODE_OF_CONDUCT.md) which
+all contributors are expected to adhere to.
+
+## Add an entry to the changelog
+
+If your contribution changes the behavior of `mosaic` (as opposed to a typo-fix
+in the documentation), please update the [`CHANGELOG.md`](CHANGELOG.md) file
+and describe your changes. This makes the release process much easier and
+therefore helps to get your changes into a new `mosaic` release faster.
+
+The top of the `CHANGELOG` contains a *"unreleased"* section with a few
+subsections (Features, Bugfixes, …). Please add your entry to the subsection
+that best describes your change.
+
+Entries follow this format:
+```
+- Short description of what has been changed, see #123 (@user)
+```
+Here, `#123` is the number of the original issue and/or your pull request.
+Please replace `@user` by your GitHub username.
+
+## Lacking API for plugin in mosaic
+
+If you have a plugin idea, but mosaic still doesn't have API required to make
+the plugin consider opening [an issue][plugin-issue] and describing your
+requirements.
+
+[plugin-issue]: https://github.com/mosaic-org/mosaic/issues/new?assignees=&labels=plugin%20system