Add basic contributing guidelines.

Author: felixarntz

.editorconfig

@@ -1,4 +1,4 @@
-# PSR-2 compliant EditorConfig.
+# PSR-12 compliant EditorConfig.
 
 root = true
 

CONTRIBUTING.md

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+# Contributing to the PHP AI Client package
+
+Thank you for your interest in contributing to the PHP AI Client package! Here you find some information on how to get started. As this repository is in its very early stages, expect more detailed instructions to come soon.
+
+## Coding standards
+
+While this project is stewarded by [WordPress AI Team](https://make.wordpress.org/ai/) members and contributors, it is a WordPress agnostic PHP package that can benefit any project in the PHP ecosystem. As such, all code must follow the [PSR-12 Coding Standards](https://www.php-fig.org/psr/psr-12/).
+
+## Guidelines
+
+- As with all WordPress projects, we want to ensure a welcoming environment for everyone. With that in mind, all contributors are expected to follow our [Code of Conduct](https://make.wordpress.org/handbook/community-code-of-conduct/).
+- All WordPress projects are [licensed under the GPLv2+](/LICENSE.md), and all contributions to the PHP AI Client package will be released under the GPLv2+ license. You maintain copyright over any contribution you make, and by submitting a pull request, you are agreeing to release that contribution under the GPLv2+ license.