docs(contributing): Add contributing guide

CONTRIBUTING.md

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+# Contributing to Medium Zoom
+
+Thank you for getting involved in Medium Zoom!
+
+## Concept
+
+This module is meant to reproduce the zoom from [Medium](http://medium.com). We want to provide a great UX (User Experience) with a lightweight vanilla JavaScript solution. Thus, this package must remain focused on this core principle, yet be extended with a composable API.
+
+## Folder structure
+
+```
+▸ cypress           The end-to-end tests written for Cypress
+▸ examples          The Medium Zoom featured examples available on CodeSandbox
+▸ src               The source code of the module
+  ▸ __tests__       The tests for the module API
+▸ stories           The stories for the Medium Zoom Storybook
+▸ website           The Medium Zoom website source code
+```
+
+## Requirements
+
+- [Node](https://nodejs.org)
+- [Yarn](https://yarnpkg.com)
+
+## Conventions
+
+### Commits
+
+This project follows the [conventional changelog](https://conventionalcommits.org/) guidelines. All commit messages should be formatted using the following scheme:
+
+```
+type(scope): description
+```
+
+## Workflow
+
+### Filing issues
+
+Reporting a bug or requesting a feature is always welcome. Feel free to [open an issue](https://github.com/francoischalifour/medium-zoom/issues/new/choose) with the according template which helps you create an effective report.
+
+### Submit code
+
+After discussing in an issue about the need to change the code, you will need to follow these steps:
+
+- [Fork the repository](https://help.github.com/articles/fork-a-repo/)
+- Clone your fork
+- Install the dependencies: `yarn`
+- For a **documentation** change:
+  - Create a branch `docs/what-you-change`
+  - Make the changes
+  - Run `yarn run format`
+- For a **bug fix**:
+  - Create a branch `fix/issue-number`
+  - [Write a test](src/__tests__/medium-zoom.test.js) to reproduce the bug (run `yarn run test`)
+  - Fix the bug in the [source code](src/medium-zoom.js)
+  - Make your test pass the previous bug
+  - Run `yarn run format` and fix problems if any
+- For a **feature**:
+  - Create a branch `feat/name-of-the-feature`
+  - Add the feature to the [source code](src/medium-zoom.js)
+  - Create a story in the [storybook](stories) showcasing the feature
+  - [Write a test](src/__tests__/medium-zoom.test.js) to ensure it's working as expected (run `yarn run test`)
+  - Run `yarn run format` and fix problems if any
+- [Create a pull request](https://help.github.com/articles/creating-a-pull-request/)
+
+We will then review your pull request!
+
+### Testing
+
+#### Unit and integration tests
+
+[Unit and integration tests](src/__tests__) with [Jest](https://jestjs.io) ensure that the API works as documented.
+
+###### Commands
+
+- Run the tests: `yarn run test`
+- Watch the tests: `yarn run test --watch`
+- Run the tests with coverage: `yarn run test --coverage`
+
+#### End-to-end tests
+
+[End-to-end tests](cypress/integration) with [Cypress](https://cypress.io) take screenshots of the stories to ensure that the zoom looks as expected.
+
+###### Commands
+
+- Open the Cypress UI: `yarn run test:cypress:open`
+- Run the Cypress tests: `yarn run test:cypress:run`
+
+### Releasing
+
+We rely on [release-it](https://github.com/webpro/release-it) to release new versions of this package.
+
+#### Release flow
+
+The release flow goes through these steps:
+
+1. Run acceptance tests
+1. Bump the project version in [`package.json`](package.json) based on [the commits](#conventions)
+1. Commit the release version
+1. Create the new Git tag for this release
+1. Push to GitHub
+1. Publish to npm
+
+#### Release steps
+
+##### Unstable version
+
+1. Make sure you're on the `next` branch
+1. Run `npm run release:next` (_do not_ use `yarn` for releasing)
+1. Follow the command-line instructions
+
+##### Stable version
+
+1. Make sure you're on the `master` branch
+1. Run `npm run release` (_do not_ use `yarn` for releasing)
+1. Follow the command-line instructions
+
+---
+
+Thank you for contributing!