reference readme as in dev 3.x, and link to older readme

README.md

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+*NOTE: This README reflects the in development version of Mirador 3. For previous versions, please reference that release's README directly. Latest 2.x release: [v.2.7.0](https://github.com/ProjectMirador/mirador/tree/v2.7.0)*
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 ## Running Mirador locally
 
 1. Run `npm install` to install the dependencies.