Add Storybook badge link

Author: mbarton

README.md

@@ -1,5 +1,7 @@
 # Digital Growth Charts React Component Library
 
+[![Storybook](https://raw.githubusercontent.com/storybookjs/brand/refs/heads/main/badge/badge-storybook.svg 'Storybook')](https://6732292d6f3624b0036f84b4-whhlqivzjh.chromatic.com/)
+
 Please go to <https://growth.rcpch.ac.uk/products/react-component/> for all documentation
 
 Issues can be raised here <https://github.com/rcpch/digital-growth-charts-react-component-library/issues>