docs: Add contributing guidelines

CONTRIBUTING.md

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+# Contributing to Planka
+
+First off, thanks for taking the time to contribute!
+
+## Code of Conduct
+
+This project and everyone participating in it is governed by the [Planka Code of Conduct](https://github.com/plankanban/planka/blob/master/CODE_OF_CONDUCT.md). By participating, you are expected to uphold this code.
+
+## How Can I Contribute?
+
+### Reporting Bugs
+
+Feel free to create a bug report as a new issue on GitHub. Before creating, please check if there is already existing one. When creating a bug report, please include as many details as possible.
+
+### Suggesting Enhancements
+
+Feel free to create an enhancement suggestion as a new issue on GitHub. Before creating, please check if there is already existing one. When creating an enhancement suggestion, please include as many details as possible.
+
+### Pull Requests
+
+Before submitting a pull request please discuss with the core team by creating or commenting in an issue on GitHub – we'd also love to hear from you in the discussions. This way we can ensure that an approach is agreed on before code is written. This will result in a much higher liklihood of your code being accepted.
+
+If you’re looking for ways to get started, here's a list of ways to help us improve Planka:
+
+- [Translation](https://github.com/plankanban/planka/issues/66) into other languages
+- Issues with [`good first issue`](https://github.com/plankanban/planka/labels/good%20first%20issue) label
+- Performance improvements, both on client and server
+- Developer happiness and documentation
+- Bugs and other issues listed on GitHub
+
+## Styleguides
+
+### Git Commit Messages
+
+Commit messages should follow the [commit message convention](https://conventionalcommits.org), so changelogs could be generated automatically by that.
+
+Additional rules:
+
+- Separate subject from body with a blank line
+- Limit the subject line to 70 characters
+- Capitalize the subject line
+- Do not end the subject line with a period
+- Use the imperative mood in the subject line
+- Use the body to explain what and why vs. how
+- Each commit should be a single, stable change
+
+### JavaScript
+
+All JavaScript code should follow this [JavaScript style guide](https://github.com/airbnb/javascript). The pre-commit hook will help you find linting errors before committing.