Update docs on previous versions

Author: wojtekmaj

packages/react-pdf/README.md

@@ -26,6 +26,7 @@ A minimal demo page can be found in `sample` directory.
 
 React-PDF is under constant development. This documentation is written for React-PDF 9.x branch. If you want to see documentation for other versions of React-PDF, use dropdown on top of GitHub page to switch to an appropriate tag. Here are quick links to the newest docs from each branch:
 
+- [v9.x](https://github.com/wojtekmaj/react-pdf/blob/v9.x/packages/react-pdf/README.md)
 - [v8.x](https://github.com/wojtekmaj/react-pdf/blob/v8.x/packages/react-pdf/README.md)
 - [v7.x](https://github.com/wojtekmaj/react-pdf/blob/v7.x/packages/react-pdf/README.md)
 - [v6.x](https://github.com/wojtekmaj/react-pdf/blob/v6.x/README.md)