CONTRIBUTING.md (#44)

* add `CONTRIBUTING.md`

* fill contributing sections

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Co-authored-by: Boog900 <54e72d8a-345f-4599-bd90-c6b9bc7d0ec5@aleeas.com>

CONTRIBUTING.md

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+# Contributing to Cuprate
+
+## Introduction
+
+Thank you for wanting to help out! Cuprate is in the stage where things are likely to change quickly, so it's recommend
+you join our [Matrix room](https://matrix.to/#/#cuprate:monero.social).
+
+## Making a PR
+
+Once you have found something you would like to work on by either looking at the open issues or joining Cuprate's [Matrix room](https://matrix.to/#/#cuprate:monero.social)
+and asking it's recommended to make your interest on working on that thing known so people don't duplicate work.
+
+When you are at a stage where you would like feedback you can open a draft PR, keep in mind that feedback may take time especially if the change is large.
+Once your PR is at the stage where you feel it's ready to go, open it for review.
+
+## Passing CI
+
+To pass CI make sure all these successfully run:
+
+- `cargo clippy --workspace --all-targets --all-features -- -D warnings`
+- `cargo fmt --all`
+- `cargo test`
+- `cargo build`
+
+## Coding guidelines
+
+- `// Comment like this.` and not `//like this`
+- Use `TODO` instead of `FIXME`
+- Avoid `unsafe`
+- Sort imports as core, std, third-party, Cuprate crates, current crate.
+- Follow the [Rust API Guidelines](https://rust-lang.github.io/api-guidelines)
+- Break the above rules when it makes sense