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CONTRIBUTING.md

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+<h1>
+    <img alt="logo" src="./docs/assets/logo-small.png" width="28px" />
+    Codeowners Validator - contributing & development
+</h1>
+
+This document contains contribution guidelines and tips for this repository. Read it to learn how to develop the Codeowners Validator project.
+
+## Table of Contents
+
+<!-- toc -->
+
+<!-- tocstop -->
+
+## Prerequisites
+
+* [Go](https://golang.org/dl/) 1.15 or higher
+* [Docker](https://www.docker.com/)
+* Make
+
+Helper scripts may introduce additional dependencies. However, all helper scripts support the `INSTALL_DEPS` environment variable flag.
+By default, this flag is set to `false`. This way, the scripts will try to use the tools installed on your local machine. This helps speed up the development process.
+If you do not want to install any additional tools, or you want to ensure reproducible script 
+results, export `INSTALL_DEPS=true`. This way, the proper tool version will be automatically installed and used. 
+
+## Contributing
+
+### Issues
+
+> **NOTE:** Before adding a new issue, check the [existing issues](https://github.com/mszostok/codeowners-validator/issues) to avoid duplicates. 
+
+If you'd like to see a certain feature added to the project, use this [feature request template](./.github/ISSUE_TEMPLATE/feature_request.md) to create an issue. 
+
+Similarly, if you spot a bug, use this [bug report template](./.github/ISSUE_TEMPLATE/bug_report.md) to let us know!
+
+### PRs
+
+> **NOTE:** Before proposing changes, check the [existing issues](https://github.com/mszostok/codeowners-validator/issues) and make sure the work has not already been started to avoid duplication. 
+
+To start contributing, follow these steps: 
+
+1. Fork the `codeowners-validator` repository.
+2. Clone the repository locally. 
+    > **TIP:** This project uses Go modules, so you can check it out locally wherever you want. It doesn't need to be checked out in `$GOPATH`.
+3. Set the `codeowners-validator` repository as upstream:
+    ```bash
+    git remote add upstream [email protected]:mszostok/codeowners-validator.git
+    ```
+4. Fetch all the remote branches for this repository:
+    ```bash
+    git fetch --all 
+    ```
+5. Set the master branch to point to upstream:
+    ```bash
+    git branch -u upstream/master master
+    ```
+
+## Dependency management
+
+This project uses `go modules` for dependency management. To install all required dependencies, use the following command:
+
+```bash
+go mod download
+```
+
+## Testing
+
+### Unit tests
+
+To run all unit tests, execute:
+
+```bash
+make test-unit
+```
+
+To generate the unit test coverage HTML report, execute: 
+
+```bash
+make test-unit-cover-html
+```
+
+> **NOTE:** The generated report opens automatically in your default browser.
+
+### Lint tests
+
+To check your code for errors, such as typos, wrong formatting, security issues, etc., execute:
+
+```bash
+make test-lint
+```
+
+To automatically fix detected lint issues, execute:
+
+```bash
+make fix-lint-issues
+```
+
+### Integration tests
+
+This project supports the integration tests that are defined in the [test](./test) package. The tests are executed against [`gh-codeowners/codeowners-samples`](https://github.com/gh-codeowners/codeowners-samples).
+
+> **CAUTION:** Currently, running the integration tests both on external PRs and locally by external contributors is not supported, as the teams used for testing are visible only to the organization members. 
+> At the moment, the `codeowners-validator` repository owner is responsible for running these tests. 
+
+## Build a binary
+
+To generate a binary for this project, execute:
+```bash
+make build
+```
+
+This command generates a binary named `codeowners-validator` in the root directory.
+
+[↑ Back to top](#table-of-contents)