Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+Smithay is open to contributions from anyone. Here are a few tips to get started if you want to participate.
+
+## Coordination
+
+Most discussion about features and their implementations takes place on github.
+If you have questions, suggestions, ideas, you can open an issue to discuss it, or add your message in an already existing issue
+if it fits its scope.
+
+If you want a more realtime discussion I (@vberger) have a Matrix room dedicated to Smithay and
+my other wayland crates: [#smithay:matrix.org](https://matrix.to/#/#smithay:matrix.org). If you don't want to
+use matrix, this room is also bridged to gitter: https://gitter.im/smithay/Lobby.
+
+## Scope
+
+Smithay attempts to be as generic and un-opinionated as possible. As such, if you have an idea of a feature that would be usefull
+for your compositor project and would like it to be integrated in Smithay, please consider whether it is in its scope:
+
+- If this is a very generic feature that probably many different projects would find usefull, it can be integrated in Smithay
+- If it is a rather specific feature, but can be framed as a special case of a more general feature, this general feature is
+  likely worth adding to Smithay
+- If this feature is really specific to your use-case, it is out of scope for Smithay
+
+## Structure
+
+Smithay aims to be a modular hierarchical library:
+
+- Functionnalities should be split into independent modules as much as possible
+- There can be dependencies in functionnalities
+- Even if most people would directly use a high-level functionnality, the lower level abstrations it is built on should
+  still be exposed independently if possible
+
+The goal is for Smithay to be a "use what you want" library, and features that are not used should have no impact on the
+application built with Smithay.