📝 CONTRIBUTING: Add contributions guidelines

Copied them over from Knigge.

Author: alexocode

CONTRIBUTING.md

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+# Contributing
+
+If you discover issues, have ideas for improvements or new features,
+please [open an issue](https://github.com/sascha-wolf/ex_union/issues)
+or submit a pull request.
+
+Make sure to follow the following guidelines when doing so.
+
+## Issue Reporting
+
+* Check that the issue has not already been reported.
+* Check that the issue has not already been fixed in the latest changes
+  (a.k.a. `main`).
+* Be clear, concise and precise in your description of the problem.
+* Open an issue with a descriptive title and a summary in grammatically correct,
+  complete sentences.
+
+## Pull Requests
+
+* Read [how to properly contribute to open source projects on GitHub](https://www.gun.io/blog/how-to-github-fork-branch-and-pull-request).
+* Fork the project.
+* Use a topic/feature branch so you're able to make additional changes later, if necessary.
+* Write [good commit messages](https://chris.beams.io/posts/git-commit/).
+* Use the same coding conventions as the rest of the project.
+* Commit and push until you are happy with your contribution.
+* Make sure to add tests for it. So we can avoid that it breaks accidentially in the future.
+* Add an entry to the [Changelog](./CHANGELOG.md) accordingly. See [changelog entry format](#changelog-entry-format).
+* Open a [pull request](https://help.github.com/articles/about-pull-requests) that relates to *only* one subject with a clear title
+  and description in grammatically correct, complete sentences.
+
+### Changelog Entry Format
+
+Here are a few examples:
+
+```
+- [#19](https://github.com/sascha-wolf/ex_union/pull/42): Fix the annoying thing ([@AmazingGitHubUser])
+- [#16](https://github.com/sascha-wolf/ex_union/pull/1337): Add caching to GitHub Actions ([@sascha-wolf])
+```
+
+* Mark it up in [Markdown syntax](https://daringfireball.net/projects/markdown/syntax).
+* The entry line should start with `- ` (a dash and a space).
+* Begin with a link to your pull request (`[#456](https://github.com/sascha-wolf/ex_union/pull/456): `)
+* Describe the core idea of the change.
+* If this is a breaking change, mark it with `**(Breaking)**`.
+* At the end of the entry, add an implicit link to your GitHub user page as `([@username])`.
+* If this is your first contribution to ExUnion, add a link definition for the implicit link to the bottom of the changelog as `[@username]: https://github.com/username`.
+
+*These guidelines were inspired by the [contribution guidelines of the rubocop project](https://github.com/rubocop/rubocop/blob/master/CONTRIBUTING.md).*