Fulfil the open source requirements for the repository (#5)

* Create CODEOWNERS, README.md and CONTRIBUTING.md files

* Update README.md file

* Update .gitignore file structure

.github/CODEOWNERS

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+* @Adyen/php

.gitignore

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+# OS
+.DS_Store
+Thumbs.db
+
+# IDE and local environment
+.vscode/
+.vagrant/
+.idea/
+
+# Packages
+vendor/

CONTRIBUTING.md

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+# Contribution guidelines
+
+### How to contribute step-by-step
+
+1. Fork the `Adyen/adyen-magento2-express-checkout` repository.
+2. Create a new branch from `develop` in your fork. This makes it easier for you to keep track of your changes.
+3. Make the desired changes to the code.
+    * If you are adding new functionality or fixing a bug, we recommend you add unit tests that cover it.
+4. Push the changes to your fork.
+5. Create a pull request to the `Adyen/adyen-magento2-express-checkout` repository.
+6. In your pull request, please describe in detail:
+    * What problem you’re solving
+    * Your approach to fixing the problem
+    * Any tests you wrote
+7. Check Allow edits from maintainers.
+8. Create the pull request.
+9. Ensure that all checks have passed.
+
+After you create your pull request, one of the code owners will review your code.
+We aim to review your request within 2-3 business days.

README.md

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+# Magento Express Checkout Module
+This module gives our customers the ability to use Apple Pay and Google Pay as an express payment method flow. This module is to be used along with the [Adyen Payment plugin for Magento 2](https://github.com/Adyen/adyen-magento2) and is not a standalone product.
+
+## Features
+* Apple Pay express checkout options on the product page, mini cart and cart.
+* Google Pay express checkout options on the product page, mini cart and cart.
+
+More express flows will be coming in 2023.
+
+## Requirements
+This plugin supports
+* Magento 2 version 2.4 and higher
+* Adyen Payment plugin for Magento 2 v8.8.0 and higher
+
+## Installation
+You can install our plugin through Composer:
+```
+composer require adyen/adyen-magento2-express-checkout
+bin/magento module:enable Adyen_ExpressCheckout
+bin/magento setup:upgrade
+bin/magento cache:clean
+```
+
+## Configuration Steps
+1. Add the payment method in your [Customer Area](https://docs.adyen.com/payment-methods#add-payment-methods-to-your-account).
+2. Make sure that Alternative payment methods are activated in your [Magento configuration](https://docs.adyen.com/plugins/magento-2/set-up-the-payment-methods-in-magento#alternative-payment-methods).
+3. In the Magento admin page, go to Alternative payment methods > Express Payments > Show Google Pay on > Select one or all of your desired options.
+4. For Apple Pay: [Use Adyen's Apple Pay Certificate to go live](https://docs.adyen.com/payment-methods/apple-pay/web-component#going-live), without designing your Apple Pay integration.
+5. For Google Pay: Set up [Google Pay](https://docs.adyen.com/payment-methods/google-pay/web-component#before-you-go-live).
+
+## Contributing
+We strongly encourage you to join us in contributing to this repository so everyone can benefit from:
+* New features and functionality
+* Resolved bug fixes and issues
+* Any general improvements
+
+Read our [**contribution guidelines**](CONTRIBUTING.md) to find out how.
+
+## Releases
+1. **Major** releases are done ONLY when absolutely required. We try to not to introduce breaking changes and do major releases as rare as possible. Current average is **yearly**.
+2. A minor or a patch release is scheduled but not limited to **once a month.**
+
+**Note: This can be subject to change based on the load and dependencies of the Integration tools team.**
+
+## Verified Payment Methods
+* Apple Pay
+* Google Pay
+
+## Support
+If you have a feature request, or spotted a bug or a technical problem, create a GitHub issue. For other questions, contact our [support team](https://support.adyen.com/hc/en-us/requests/new?ticket_form_id=360000705420).
+
+## License
+MIT license. For more information, see the [LICENSE](LICENSE) file.