Create CONTRIBUTING.md

Partly inspired by and copied from https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md

CONTRIBUTING.md

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+# How to contribute
+
+SAUCE heavily uses the
+successful Git branching model](http://nvie.com/posts/a-successful-git-branching-model/)
+by Vincent Driessen
+([summary figure](http://nvie.com/files/Git-branching-model.pdf)).
+His [git-flow extension](https://github.com/nvie/gitflow) makes it easier to employ this model.
+
+*Please* just submit your _feature_ branches for pull requests.
+
+---
+The following sections are mostly copied from https://github.com/puppetlabs/puppet/blob/master/CONTRIBUTING.md
+---
+
+## Getting Started
+
+* If it's a bug or something worth discussing first, submit a ticket for your issue,
+  assuming one does not already exist. You can skip that for simple new features
+  since discussion can also happen on the pull request.
+  * Clearly describe the issue including steps to reproduce when it is a bug.
+  * Make sure you fill in the earliest version that you know has the issue.
+* Fork the repository on GitHub
+
+## Making Changes
+
+* Create a _feature_ branch (also called topic branch) from where you want to base your work
+  (see the note on git-flow above).
+  * This is usually the `develop` branch.
+  * If you are certain your fix must be on a _release_ branch, you should use a _hotfix_ branch.
+  * Please avoid working directly on the `develop` and `master` branches.
+* Make commits of logical units.
+* Check for unnecessary whitespace with `git diff --check` before committing.
+* Make sure your commit messages are in the proper format.
+* If you can, please add tests for your changes (even broken tests are better than no tests).
+
+## Submitting Changes
+
+* Push your changes to a feature branch in your fork of the repository.
+* Submit a pull request to the repository.