added contributing md

CONTRIBUTING.md

@@ -0,0 +1,68 @@
+# How to contribute to AlmKanal
+
+We always welcome contributions to AlmKanal.
+
+## General rules
+
+## Setting up the development environment
+We have decided to use the following tools:
+
+1. [Pixi](https://pixi.sh) for dependency management.
+2. [Ruff](https://docs.astral.sh/ruff/) for linting and formatting.
+3. [Hatch](https://hatch.pypa.io/) for packaging and distribution.
+
+### Install the software
+The first thing to do is to install [Pixi](https://pixi.sh) which manages all the rest for us.
+
+Check the [installation guide on their website](https://pixi.sh/latest/#installation)
+
+Afterwards, just run `pixi install` in the root folder of the repository and
+it's going to take care of the rest.
+
+This is going to create a so-called "environment", i.e. a folder in which all
+the dependencies are installed. To use the environment, you need to run `pixi shell`.
+
+### Optional: Install the precommit hook
+We have a precommit hook that runs the linter and the formatter before every commit.
+To install it, run `pre-commit install`.
+
+## How to do common tasks
+### Add dependencies for the project
+
+There are two kinds of dependencies: runtime and development dependencies. Both
+are managed in the `pyproject.toml` file.
+
+* Runtime dependencies are added to the `[project]` -> `dependencies` section.
+* Development dependencies can be found in the `[tool.pixi.dependencies]` section.
+  * You can use the `pixi add` command to add a new development dependency.
+
+After you have edited the `pyproject.toml` file, run `pixi install` to install the new dependencies.
+
+### Linting and formatting
+Linting and formatting are done with [Ruff](https://docs.astral.sh/ruff/).
+These rules are going to be enforced by CI in the future.
+
+If you want to check your code manually, just run `pixi run lint`.
+
+### Building and distributing the package
+We use [Hatch](https://hatch.pypa.io/) to build and distribute the package. It
+is also used to manage the versioning of the package.
+
+Eventually, building and distributing the package will be done automatically by CI.
+But if you want to test it, you can just use `hatch build` and it is going to
+create a wheel file in the `dist` folder.
+
+### Managing version
+We use [Hatch](https://hatch.pypa.io/) to manage the version of the package.
+
+The single source of truth for the version is found in the `almkanal/__version__.py` file.
+
+You can use hatch to view the current version with `hatch version`. If you want
+to create a new version, hatch increments the version number for you and also creates
+a tag for you.
+
+* `hatch version dev` creates a new development version.
+* `hatch version fix` creates a new patch version.
+* `hatch version minor` creates a new minor version.
+
+Just make sure to also push the tag to the repository with `git push --tags`.