Update main README with a link to the new Animal-AI version.

README.md

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+**UPDATE: Animal-AI v3 is now up [on a separate repo](https://github.com/mdcrosby/animal-ai.git).** The below repository contains the codebase for the competition that was ran in 2019. It is not under active maintenance but issues are still monitored to some extent. 
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 # Animal-AI 2.0.0
 
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