Adding deprecation warning (#456)

Co-authored-by: Ole Martin Handeland <git@olemartin.org>

Author: olemartinorg

README.md

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 # Altinn Design System
 
+⚠️⚠️⚠️ _**DEPRECATED**_ ⚠️⚠️⚠️  
+**The [design system v2](https://github.com/Altinn/altinn-design-system) ([npm](https://www.npmjs.com/package/@altinn/altinn-design-system)) supersedes this project. Consider moving your project to rely on the newer design system instead of this one.**
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+<hr>
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 This design system is based on [Pattern Lab](https://patternlab.io/) code and [atomic design principles](http://bradfrost.com/blog/post/atomic-web-design/).
 The [repo](https://github.com/Altinn/DesignSystem) contains the source code and configuration for building and running the design system locally and for creating distribution files.