docs(contributing): Add CONTRIBUTING.md

This outlines a basic CONTRIBUTING.md with a few tips on development as well as
important information to be aware of.

Signed-off-by: Christina Sørensen <christina@cafkafk.com>

CONTRIBUTING.md

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+<!--
+SPDX-FileCopyrightText: 2023 Christina Sørensen
+SPDX-FileContributor: Christina Sørensen
+
+SPDX-License-Identifier: AGPL-3.0-only
+-->
+
+# Contributing to rime
+
+If you'd like to contribute to rime, there are several things you should make
+sure to familiarize yourself with first.
+
+- Code of conduct [Contributor Covenant Code of Conduct](CODE_OF_CONDUCT.md)
+- Usage of the [REUSE standard](https://reuse.software/)
+- Requirement of conformance to [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/)
+- Requirement of conformance to Semantic Versioning](https://semver.org/)
+- The [Security Policy](SECURITY.md)
+- [Free and Open Source (FOSS) software](https://www.gnu.org/philosophy/free-sw.en.html)
+
+## Hacking on rime
+
+Rime is a Nix project, and further a project for Nix Flakes. Therefore, it is
+expected that you have a version of Nix installed with the "experimental"
+feature flakes enabled. Further, to make hacking on rime as easy as possible
+for yourself, you'd do yourself a favor to install [direnv](https://direnv.net/)
+(but this is optional).
+
+When you enter the rime repository, if you have `direnv` installed, you'll be
+prompted to allow it with `direnv allow`. Doing this will save you from having
+to manually enter the development environment each time you open the folder. If
+you don't have direnv installed however, you can run `nix develop` in a pinch,
+to enter the direnv.
+
+The development environment includes basic checks of conformance to
+conventional commits, cargo clippy lints, REUSE compliance, and much more.
+
+It also contains a pre-commit-hook making it a lot easier not to make potential
+mistakes that will unnescesarrily delay getting your PRs accepted.
+
+## Before submitting a PR
+
+Please make sure that the thing you worked on... actually works. Make sure to
+also add how you ensured this in the PR description. Further, it's expected
+that you do your best to check for regressions. We currently don't have any
+test suite, so you'll have to use your best judgment. 
+
+Before submitting, you MUST have run `nix flake check` and ensured that all
+issues are addressed. For formatting issues, `nix fmt` will format the code for
+you. Most clippy issues can be resolved with `cargo clippy --fix` (although it
+might be eduational to fix them yourself). If you have reuse issues, you can
+run the following command to annotate your code:
+
+```
+reuse annotate --license AGPL-3.0-only --contributor "<Your name>" --copyright "<Your name>" <file(s) you'd like to annotate>
+```
+
+For certain binary files or files that can't easily contain comments for
+whatever reason, the `--force-dot-license` can help.
+
+Remeber that no one here is an employee, and treat everyone with respect, as
+the code of conduct specifies. Also remember to be patient if it takes a while
+to get a response on your PR. Usually it doesn't, but there's only so many
+hours in a day, and if possible, there would be no delay. The delay alone is
+evidence of it's own nescessity.
+
+---
+
+Have fun hacking on rime!