Add common README sections (#30)

Author: dantleech

README.md

@@ -86,3 +86,16 @@ About this project
 
 This library is part of the [phpactor](https://github.com/phpactor/phpactor)
 project.
+
+Contributing
+------------
+
+This package is open source and welcomes contributions! Feel free to open a
+pull request on this repository.
+
+Support
+-------
+
+- Create an issue on the main [Phpactor](https://github.com/phpactor/phpactor) repository.
+- Join the `#phpactor` channel on the Slack [Symfony Devs](https://symfony.com/slack-invite) channel.
+