Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing to zerOS
+
+## General notes
+
+As far as you're kind to contributors / other users and you respect the [code of conduct](CODE_OF_CONDUCT.md), you're very welcome to join this long development journey.
+We accept any contributions, from bug fixes to complete new features (as far as it is interesting and not _too_ crazy).
+
+All new contributions should be made by github pull requests.
+
+## zerOS kernel
+
+### Coding guidelines
+
+#### Style
+
+So far, no particular style is _really_ enforced. However, it would be very appreciated, as the codebase will grow, to stick to the `.clang-format` we provide.
+Any request to modify the style must be accompanied by a valid rationale.
+
+#### APIs
+
+Names of functions from kernel subsystems APIs should be prefixed with `zerOS_`. You're otherwise free to name other `static` functions as you see fit.