meta: add CONTRIBUTING.md

CONTRIBUTING.md

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+This document is a guide for contributions to the WGPU project.
+
+## Welcome!
+
+First of all, welcome to the WGPU community! 👋 We're glad you want to
+contribute. If you are unfamiliar with the WGPU community as an organization, we
+recommend you read [`GOVERNANCE.md`] for an overview of how it works.
+
+[`GOVERNANCE.md`]: ./GOVERNANCE.md
+
+## Talking to other humans in the WGPU project
+
+The WGPU project has multiple ways to engage:
+
+- [GitHub discussions]: TODO: not sure when to use this over Matrix and issues.
+  TBH.
+- The Matrix channel [`wgpu:matrix.org`](https://matrix.to/#/#wgpu:matrix.org)
+  is dedicated to informal chat about contributions the project. It is
+  particularly useful for:
+  - …saying hello, and introducing yourself.
+  - …validating contributions (i.e., determining if they'll be accepted,
+    ensuring your approach is correct, making sure you aren't wasting effort,
+    etc.)
+  - …setting expectations for contributions 
+
+  Notification in Matrix can sometimes be unreliable. Feel free to explicitly tag
+  people from whom you would like attention (but don't abuse it!), and follow-up
+  after a day or so if you do not get a response to your messages.
+
+- [GitHub issues] are for action items that the community needs to prioritize.
+  These could be items of work resolved by pull requests, decisions to make with
+  a formal outcome, or collections of issues needed for a specific feature or
+  use case (AKA `[meta]` issues).
+- [GitHub pull requests]: TODO
+
+[GitHub discussions]: https://github.com/gfx-rs/wgpu/discussions
+[GitHub issues]: https://github.com/gfx-rs/wgpu/issues
+[GitHub pull requests]: https://github.com/gfx-rs/wgpu/pulls
+
+## Contributing to WGPU
+
+Community response to contributions are, in general, prioritized based on their
+relevance to WGPU's mission and decision-making groups' interest (see
+[`GOVERNANCE.md`]). 
+
+### "What can I work on?"
+
+TODO
+
+### What to expect when you file an issue
+
+TODO
+
+### What to expect when you submit a PR
+
+TODO: It is strongly recommended that you validate your contributions before you make
+significant efforts…