docs: add CONTRIBUTING.md

CONTRIBUTING.md

@@ -0,0 +1,78 @@
+# Contributing guide
+
+Contributions are more than welcome!
+
+Please don't forget to add your changes to the "Unreleased" section of [the changelog](./CHANGELOG.md)
+(if applicable).
+
+## Commit messages
+
+This project uses [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/).
+
+## Development
+
+I use
+
+- [`nix`](https://nixos.org/download.html#download-nix) for development and testing.
+- [`stylua`](https://github.com/JohnnyMorganz/StyLua),
+  [`.editorconfig`](https://editorconfig.org/),
+  and [`alejandra`](https://github.com/kamadorueda/alejandra)
+  for formatting.
+- [`luacheck`](https://github.com/mpeterv/luacheck),
+  and [`markdownlint`](https://github.com/DavidAnson/markdownlint),
+  for linting.
+- [`sumneko-lua-language-server`](https://github.com/sumneko/lua-language-server/wiki/Diagnosis-Report#create-a-report)
+  for static type checking.
+
+### Running tests
+
+This plugin uses [`busted`](https://lunarmodules.github.io/busted/) for testing.
+
+The best way to run tests is with Nix (see below),
+because this includes tests that take different
+envrionments into account (e.g. with/without `rust-analyzer`, `cargo`, ...).
+
+If you do not use Nix, you can run a basic version of the test suite using
+`luarocks test`.
+For more information, see the [neorocks tutorial](https://github.com/nvim-neorocks/neorocks#without-neolua).
+
+### Development using Nix
+
+> **Note**
+>
+> [Flakes](https://nixos.wiki/wiki/Flakes) must be enabled.
+
+To enter a development shell:
+
+```console
+nix develop
+```
+
+To apply formatting, while in a devShell, run
+
+```console
+pre-commit run --all
+```
+
+If you use [`direnv`](https://direnv.net/),
+just run `direnv allow` and you will be dropped in this devShell.
+
+## Tests
+
+To run tests locally
+
+```console
+nix build .#checks.<your-system>.haskell-tools-test --print-build-logs
+```
+
+For formatting and linting:
+
+```console
+nix build .#checks.<your-system>.formatting --print-build-logs
+```
+
+If you have flakes enabled and just want to run all checks that are available, run:
+
+```console
+nix flake check --print-build-logs
+```