doc: add contribution guide

Author: Lauric Desauw

contributing.md

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+# Contributing to PowerAPI
+
+Thank you for your interest in contributing to PowerAPI! There are multiple ways to contribute, and we appreciate all contributions.
+
+# Ways to contribute
+
+Pull requests should be made on the pre-release branch. This way they will be
+included in the next stable release (available on master).
+Each pull request should pass the tests and be correctly linted to pass the CI.
+
+Each pull request have to be reviewed before beeing accepted. Some modifications can be
+asked by the projet's managers.
+
+Here is different ways to contribute to PowerAPI.
+
+## Bug report
+
+If you are working with PowerAPI and you encounter a bug, please let us know.
+You are welcome to submit an issue which describe the bug and it context with a
+small test or a code snippet that reproduce the issue.
+
+## Bug Fixes
+
+If you are interested to contribute to PowerAPI you can fixes reported bugs.
+They are listed in the issues with the label _bug_
+
+Your pull request should contain a test that reproduce the bug, if the test do
+not already exist, and the bug fix.
+Please submit one pull request for each bug fix.
+
+## Patches
+
+You can submit bigger contributions in the form of patches.
+A patch should :
+
+- include unit tests
+- be an isolated changes. If you have uncorelated changes you should submit multiple pull requests to make it easier for us to review