[TASK] Link the contribution guidelines from the README (#661)

Part of #489

Author: oliverklee

README.md

@@ -779,6 +779,13 @@ classDiagram
     ValueList --> "*" Value : aComponents
 ```
 
+## Contributing
+
+Contributions in the form of bug reports, feature requests, or pull requests are
+more than welcome. :pray: Please have a look at our
+[contribution guidelines](.github/CONTRIBUTING.md) to learn more about how to
+contribute to PHP-CSS-Parser.
+
 ## Contributors/Thanks to
 
 * [oliverklee](https://github.com/oliverklee) for lots of refactorings, code modernizations and CI integrations