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CONTRIBUTING.md

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+# Contributing to ol-ext
+
+Thanks for your interest in contributing to ol-ext.
+
+## Submitting Bug Reports or Asking Questions
+
+Please use the [GitHub issue tracker](https://github.com/Viglino/ol-ext/issues). 
+Before creating a new issue, do a quick search to see if the problem has been reported already.
+
+## Contributing Code
+
+See [`DEVELOPING.md`](https://github.com/Viglino/ol-ext/DEVELOPING.md) to learn how to get started developing.
+
+Clone the repository and [pull requests](https://help.github.com/articles/using-pull-requests). Make sure
+that your pull request follows our pull request guidelines below before submitting it.
+
+This page describes what you need to know to contribute code to OpenLayers as a developer.
+
+## Contributor License Agreement
+
+Your contribution will be under our [license](https://raw.githubusercontent.com/Viglino/ol-ext/master/licence.txt) 
+as per [GitHub's terms of service](https://help.github.com/articles/github-terms-of-service/#6-contributions-under-repository-license).
+
+The CeCILL licence is a french BSD licence.
+
+## Pull request guidelines
+
+Your pull request must:
+* Follow the naming convention in the DEVELOPPING.md
+* Address a single issue or add a single item of functionality.
+* Use clear commit messages.
+* Be possible to merge automatically.