Add Contributing guide

CONTRIBUTING.md

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+How to contribute
+=================
+
+Thanks for your interest in contributing to Kinto!
+
+## Reporting Bugs
+
+Report bugs at https://github.com/Kinto/kinto-http.py/issues/new
+
+If you are reporting a bug, please include:
+
+ - Any details about your local setup that might be helpful in troubleshooting.
+ - Detailed steps to reproduce the bug or even a PR with a failing tests if you can.
+
+
+## Ready to contribute?
+
+### System Requirements
+
+Depending on the platform and chosen configuration, some libraries or extra services are required.
+
+The following commands will install necessary tools for cryptography and Python packaging like Virtualenv.
+
+#### Linux
+On Debian / Ubuntu based systems:
+
+```bash
+apt-get install libffi-dev libssl-dev python-dev python-virtualenv
+```
+
+#### OS X
+Assuming [brew](http://brew.sh/) is installed:
+
+```bash
+brew install libffi openssl pkg-config python
+
+pip install virtualenv
+```
+
+### Getting Started
+
+ -  Fork the repo on GitHub and clone locally:
+
+```bash
+git clone git@github.com:Kinto/kinto-http.py.git
+git remote add {your_name} git@github.com:{your_name}/kinto-http.py.git
+```
+
+## Testing
+
+ -  `make tests-once` to run the test with the current venv.
+ -  `make tests` to run all the tests (with Py2 and Py3, flake8 and functional tests)
+
+You may need to use `make run-kinto` before running the functional tests.
+If you want to run the functional tests only, you can use `make functional`.
+
+## Submitting Changes
+
+```bash
+git checkout master
+git pull origin master
+git checkout -b issue_number-bug-title
+git commit # Your changes
+git push -u {your_name} issue_number-bug-title
+```
+
+Then you can create a Pull-Request.
+Please create your pull-request as soon as you have one commit even if it is only a failing tests. This will allow us to help and give guidance.
+
+You will be able to update your pull-request by adding commit in your branch.