Add contributing guide

CONTRIBUTING.md

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+# CONTRIBUTING
+
+## Introduction
+
+[Since ~February 2022, the Zed Industries team has been exclusively using Zed to build Zed](https://x.com/nathansobo/status/1497958891509932035). We've built these tools to specifically address our own issues and frustrations with the current state of collaborative coding. These are not features we've built to simply look flashy, we work in channels every week, aggressively dogfooding our own tools.
+
+![Staff usage of channels (metrics were not being collected before August, 2023)](./assets/screenshots/staff_usage_of_channels.png)
+
+While we still have improvements to make, we believe we've sanded down a lot of the sharp edges and that experience is both smooth and enjoyable - one that gets you as close to hypothetically sitting next to your teammates as possible, even if you're potentially on different sides of the globe. We want to continue working this way amongst ourselves, but we are extremely excited to work with *you* in this way. We invite you to contribute to Zed *through* Zed.
+
+If you're new to Zed's channels, here's a guide [link to up-to-date docs] to help bring you up to speed.
+
+## Contribution ideas
+
+*If you already have an idea of what you'd like to contribute, you can skip this section.*
+
+- Our public roadmap [include link] shows the largest, most-wanted features we plan to add to Zed.
+- Our [Top-Ranking Issues issue](https://github.com/zed-industries/community/issues/52) shows the most popular feature requests and issues, as voted on by the community.
+
+*If you are a plugin developer looking to contribute by building out the Zed ecosystem, have a look at these [issues](https://github.com/zed-industries/community/issues?q=is%3Aopen+is%3Aissue+label%3A%22potential+plugin%22+sort%3Areactions-%2B1-desc).*
+
+## Proposal & Discussion
+
+Once you have an idea of what you'd like to contribute, you'll want to communicate this to the team. Find a public channel [link to list of all public channels] that is relevant to your contribution, check the channel notes to see which Zed team members typically work in that channel, and post a message in the chat. If you're not sure which channel is best, you can post in the <whatever-general> channel. *Please wait to begin working on your contribution until you've received feedback from the team. Turning down a contribution that was not discussed beforehand is a bummer for everyone.*
+
+## Implementation & Help
+
+Once approved, feel free to begin working on your contribution. If you have any questions, you can post in the channel you originally proposed your contribution in, or you can post in the <whatever-general> channel. If you need help, reach out to a Zed teammate - we're happy to pair with you to help you learn the codebase and get your contribution merged.
+
+Reviewing code in a pull request, after the fact, is hard and tedious - the team generally likes to build trust and review code through pair programming. We'd prefer have conversations about the code, through Zed, while it is being written, so decisions can be made in real-time and less time is spent on fixing things after the fact. Ideally, GitHub is only used to merge code that has already been discussed and reviewed in Zed.
+
+---
+
+Other things to mention here
+- [ ] Etiquette
+- [ ] CLA
+
+Things to do:
+- [ ] Put names devs who "own" each channel in the channel notes