Add common README sections (#9)

Author: dantleech

README.md

@@ -2,3 +2,16 @@ Phactor Completion Extension
 ============================
 
 ![CI](https://github.com/phpactor/completion-extension/workflows/CI/badge.svg)
+
+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.
+