Update readme to specify that Storybook v6 requires an earlier version of this package

[jhancock532]

The latest version of this package seems to have dropped support for Storybook v6. To avoid confusion, a line of text could be added to the readme, recommending users to install an earlier package version if they use an earlier storybook instance.

For reference: storybook-addon-pseudo-states: 1.15.5 worked for me on Storybook v6.5.16

[gulagha]

Thanks for raising this! Helped me to save some time.

[kharrop]

This also helped save me a bunch of time, thank you for providing a working version - this works for 6.2.9 for what it's worth