Add CONTRIBUTING guide

CONTRIBUTING.md

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+# Contributing
+
+Thank you for considering to contribute to `hexyl`!
+
+
+
+## Add an entry to the changelog
+
+If your contribution changes the behavior of `hexyl` (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 proess much easier and
+therefore helps to get your changes into a new `hexyl` 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.
+
+
+## Adding a new feature
+
+Please consider opening a [ticket](https://github.com/sharkdp/hexyl/issues/new)
+first in order to give us a chance to discuss the feature first.