docs: added contributing doc

docs/docs/contributing.md

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+---
+sidebar_position: 6
+---
+
+# Contributing
+
+Contributions to Gilt are very welcome, but we ask that you read this document
+before submitting a PR.
+
+:::note
+
+This document applies to the core [Gilt][] repository.
+
+:::
+
+## Before you start
+
+- **Check existing work** - Is there an existing PR? Are there issues discussing
+ the feature/change you want to make? Please make sure you consider/address
+ these discussions in your work.
+- **Backwards compatibility** - Will your change break existing Giltfiles? It is
+ much more likely that your change will merged if it backwards compatible. Is
+ there an approach you can take that maintains this compatibility? If not,
+ consider opening an issue first so that API changes can be discussed before
+ you invest your time into a PR.
+
+## 1. Setup
+
+- **Go** - Gilt is written in [Go][]. We always support the latest two major
+ Go versions, so make sure your version is recent enough.
+- **Node.js** - [Node.js][] is used to host Gilt's documentation server
+ and is required if you want to run this server locally.
+
+## 2. Making changes
+
+- **Code style** - Try to maintain the existing code style where possible. Go
+ code should be formatted by [`gofumpt`][gofumpt] and linted using
+ [`golangci-lint`][golangci-lint]. This style is enforced by our CI to ensure
+ that we have a consistent style across the project. You can use the
+ `task fmt:check` command to lint the code locally and the `task fmt` command
+ to automatically fix any issues that are found.
+- **Documentation** - Ensure that you add/update any relevant documentation. See
+ the [updating documentation](#updating-documentation) section below.
+- **Tests** - Ensure that you add/update any relevant tests and that all tests
+ are passing before submitting the PR. See the [writing tests](#writing-tests)
+ section below.
+
+### Running your changes
+
+To run Gilt with working changes, you can use `go run main.go overlay`.
+
+### Updating documentation
+
+Gilt uses [Docusaurus][] to host a documentation server. The code for
+this is located in the Gilt repository. This can be setup and run locally
+by using `task docs:start` (requires `nodejs` & `yarn`). All content is written
+in Markdown and is located in the `docs/docs` directory.
+
+### Writing tests
+
+When making a changes, consider whether new tests are required. These tests
+should ensure that the functionality you are adding will continue to work in the
+future. Existing tests may also need updating if you have changed Gilt's
+behavior.
+
+You may also consider adding unit tests for any new functions you have added.
+The unit tests should follow the Go convention of being location in a file named
+`*_test.go` in the same package as the code being tested.
+
+Integration tests are located in the `tests` directory and executed by [Bats][].
+
+## 3. Committing your code
+
+Try to write meaningful commit messages and avoid having too many commits on the
+PR. Most PRs should likely have a single commit (although for bigger PRs it may
+be reasonable to split it in a few). Git squash and rebase is your friend!
+
+If you're not sure how to format your commit message, check out [Conventional
+Commits][]. This style is enforced, and is a good way to make your commit
+messages more readable and consistent.
+
+## 4. Submitting a PR
+
+- **Describe your changes** - Ensure that you provide a comprehensive
+ description of your changes.
+- **Issue/PR links** - Link any previous work such as related issues or PRs.
+ Please describe how your changes differ to/extend this work.
+- **Examples** - Add any examples or screenshots that you think are useful to
+ demonstrate the effect of your changes.
+- **Draft PRs** - If your changes are incomplete, but you would like to discuss
+ them, open the PR as a draft and add a comment to start a discussion. Using
+ comments rather than the PR description allows the description to be updated
+ later while preserving any discussions.
+
+## FAQ
+
+> I want to contribute, where do I start?
+
+All kinds of contributions are welcome, whether its a typo fix or a shiny new
+feature. You can also contribute by upvoting/commenting on issues or helping to
+answer questions.
+
+> I'm stuck, where can I get help?
+
+If you have questions, feel free open a [Discussion][] on GitHub.
+
+[Gilt]: https://github.com/retr0h/go-gilt
+[Go]: https://go.dev
+[Node.js]: https://nodejs.org/en/
+[gofumpt]: https://github.com/mvdan/gofumpt
+[golangci-lint]: https://golangci-lint.run
+[Docusaurus]: https://docusaurus.io
+[Discussion]: https://github.com/retr0h/go-gilt/discussions
+[Conventional Commits]: https://www.conventionalcommits.org
+[Bats]: https://github.com/bats-core/bats-core