Contributing document draft

Signed-off-by: Joffrey F <joffrey@docker.com>

CONTRIBUTING.md

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+# Contributing guidelines
+
+Thank you for your interest in the project. We look forward to your
+contribution. In order to make the process as fast and streamlined as possible,
+here is a set of guidelines we recommend you follow.
+
+## Reporting issues
+
+We do our best to ensure bugs don't creep up in our releases, but some may
+still slip through. If you encounter one while using docker-py, please create
+an issue [in the tracker](https://github.com/docker/docker-py/issues/new) with
+the following information:
+
+- docker-py version, docker version and python version
+```
+pip freeze | grep docker-py && python --version && docker version
+```
+- The issue you're encountering including a stacktrace if applicable
+- If possible, steps or a code snippet to reproduce the issue
+
+To save yourself time, please be sure to check our
+[documentation](http://docker-py.readthedocs.org/) and use the
+[search function](https://github.com/docker/docker-py/search) to find out if
+it has already been addressed, or is currently being looked at.
+
+## Submitting pull requests
+
+Do you have a fix for an existing issue, or want to add a new functionality
+to docker-py? We happily welcome pull requests. Here are a few tips to make
+the review process easier on both the maintainers and yourself.
+
+### 1. Sign your commits
+
+Please refer to the ["Sign your work"](https://github.com/docker/docker/blob/master/CONTRIBUTING.md#sign-your-work)
+paragraph in the Docker contribution guidelines.
+
+### 2. Make sure tests pass
+
+Before we can review your pull request, please ensure that nothing has been
+broken by your changes by running the test suite. You can do so simply by
+running `make test` in the project root. This also includes coding style using
+`flake8`
+
+### 3. Rebase proactively
+
+It's much easier to review a pull request that is up to date against the
+current master branch.
+
+### 4. Notify thread subscribers when changes are made
+
+GitHub doesn't notify subscribers when new commits happen on a PR, and
+fixes or additions might be missed. Please add a comment to the PR thread
+when you push new changes.
+
+### 5. Two maintainers LGTM are required for merging
+
+Please wait for review and approval of two maintainers, and respond to their
+comments and suggestions during review.
+
+### 6. Add tests
+
+Whether you're adding new functionality to the project or fixing a bug, please
+add relevant tests to ensure the code you added continues to work as the
+project evolves.
+
+### 7. Add docs
+
+This usually applies to new features rather than bug fixes, but new behavior
+should always be documented.
+
+### 8. Ask questions
+
+If you're ever confused about something pertaining to the project, feel free
+to reach out and ask questions. We will do our best to answer and help out.