Update README to account for removal of ResourceRouter and DataSourceRouter

[paddycarver]

Does this documentation exist?

• This is new documentation
• This is an enhancement to existing documentation

Where would you expect to find this documentation?

• On terraform.io
• In the GoDoc for this module
• In this repo as a markdown file
• Somewhere else

Details

The README currently instructs developers to use the ResourceRouter and DataSourceRouter types, but these types no longer exist after #24

[github-actions]

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