added v4 advertisement to README

README.md

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+> [!IMPORTANT]
+> QuEST _**v4**_ is coming very soon with a new API, which is both greatly extended and accelerated. See some of the new features listed on the [Discussions](https://github.com/QuEST-Kit/QuEST/discussions) page. If you are beginning a new project with QuEST, consider using the in-development [v4](https://github.com/QuEST-Kit/QuEST/tree/v4) source. While this version is mostly complete, it currently lacks documentation and full unit tests. Please contact Tyson at `[email protected]` to understand whether the v4 draft is safe to use in your application, and for help getting it running.
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 <!-- banner -->
 [<img src="https://github.com/QuEST-Kit/QuEST/blob/master/doxyconfig/banner.png?raw=true" alt="The QuEST logo" width=500>](https://quest.qtechtheory.org)