New contribution guidelines

Updated README.md and added CONTRIBUTING.md

CONTRIBUTING.md

@@ -0,0 +1,20 @@
+# Contribution guidelines
+
+## How to contribute step-by-step
+
+1. Fork the `Adyen/adyen-php-api-library` 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-php-api-library` 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

@@ -132,5 +132,13 @@ By default the test will then be skipped. If you have these permissions fill in
 ## Support
 If you have any problems, questions or suggestions, create an issue here or send your inquiry to [email protected].
 
+## 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.
+
 ## Licence
 MIT license. For more information, see the LICENSE file.