New contribution guidelines

Updated README.md and added CONTRIBUTING.md

CONTRIBUTING.md

+# Contribution guidelines
+
+## How to contribute step-by-step
+
+1. Fork the `Adyen/adyen-magento2` 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` 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

@@ -14,9 +14,14 @@ Inside Adyen toggle the following settings on inside the API and Responses secti
 ## Requirements
 This plugin supports Magento2 version 2.2.8 and higher.
 
-## Collaboration
-We commit all our new features directly into our GitHub repository.
-But you can also request or suggest new features or code changes yourself!
+## 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**](contribute) to find out how.
+
 
 ## Installation
 You can install our plugin through Composer: