docs: update contributing guidelines (#640)

Author: bashbunni

README.md

@@ -234,6 +234,12 @@ following requirements:
 
 Thank you!
 
+## Contributing
+
+See [contributing][contribute].
+
+[contribute]: https://github.com/charmbracelet/bubbles/contribute
+
 ## Feedback
 
 We’d love to hear your thoughts on this project. Feel free to drop us a note!