Contributing: add guidelines.

.github/ISSUE_TEMPLATE.md

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+# This is to discuss a [bug|feature|question]
+
+In case of a bug:
+
+## What version are you using?
+
+## What are the steps to reproduce the issue?
+
+## What is the expected behaviour?
+
+## What is the actual behaviour?
+
+In case of a feature:
+
+## Describe the feature
+
+In case of a question:
+
+## Write down your question, possibly with an example

CHANGELOG.md

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+# Changelog
+
+## v1.0.0 - 2016-08-08
+First official release.

CONTRIBUTING.md

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+# Contributing to Experiment
+
+## Did you find a bug?
+
+When [reporting bugs](https://github.com/jelmersnoeck/experiment/issues/new), try to be as thorough as possible:
+- What version are you using?
+- What are the steps to reproduce the issue?
+- What is the expected behaviour?
+- What is the actual behaviour?
+
+When creating a new issue, a template will be injected where you can choose
+wether it is a bug report, a feature request or a question.
+
+## Did you write a patch?
+
+Bug fixes and code improvements are always welcome (thanks for taking the time!).
+
+New features are welcome as well, but they will be evaluated per case. A general
+rule is to not generate external dependencies. If you'd rather not risk spending
+time on a patch to possibly see it turned down later on, please start a
+discussion by opening a [new issue](https://github.com/jelmersnoeck/experiment/issues/new) where you state this is a feature request.
+
+Don't forget to add your changes to the [changelog](CHANGELOG.md).