doc: Add contribution guidelines

CONTRIBUTING.md

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+# Contribution Guidelines
+
+## Getting Started
+
+1. Fork the repository & clone your fork
+2. Install dependencies with `yarn install` _(uses Yarn v1)_
+3. Build the sources with `yarn build`
+
+## Running tests
+
+1. Run `yarn test` to run all tests. This includes:
+
+- Unit tests (`yarn test:unit`)
+- Typechecking (`yarn test:types`)
+- Integration tests (`yarn test:integration`)
+
+The [integration test suite](./src/tests/integration.test.ts) runs against a local SQLite database.
+It is copied from `./prisma/db.test.sqlite` to `./prisma/db.integration.sqlite` before each test runs.
+
+The integration test suite runs twice:
+
+- Once for the middleware API
+- Once for the client extension API
+
+## Contributing
+
+First off, thanks a lot for your help!
+
+### Commit messages
+
+This package uses [semantic-release](https://semantic-release.gitbook.io/semantic-release) to manage versions.
+
+Therefore, make sure your commit messages follow the [Conventional Commits](https://www.conventionalcommits.org/en/v1.0.0/) format.
+There is a pre-commit hook in place that lints commit messages to ensure they follow the format.
+
+### Pull requests
+
+Please [open an issue](https://github.com/47ng/prisma-field-encryption/issues/new/choose) before submitting a pull request, so we can iterate
+on the suggestion and solution.
+
+If your pull request fixes an issue or adds a functionality, please add
+a test case **at the end** of the [integration test suite](./src/tests/integration.test.ts). Each test runs in sequence, and shares the same database, so its state must be kept in mind.