Update README.md (microsoft#11774)

Updating `main` branch's README.md to point to new MRTK organization.

README.md

@@ -1,3 +1,23 @@
+> [!IMPORTANT]  
+> With the creation of the new <a href="https://github.com/MixedRealityToolkit/">Mixed Reality Toolkit</a> organization, there now exists two MRTK repositories,
+> one repository for version 3+ and a legacy one for version 2.
+> <br/><br/>
+> <strong>MRTK v3+</strong>
+> <br/><br/>
+> New versions of the Mixed Reality Toolkit will be released by the <a href="https://github.com/MixedRealityToolkit/">Mixed Reality Toolkit</a>
+> organization using a repository at <a href="https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity">https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity</a>.
+> Visit this <a href="https://github.com/MixedRealityToolkit/MixedRealityToolkit-Unity">repository</a> for the latest version of the MRTK project, and when
+> creating new issues or discussion topics for MRTK version 3 or later.
+> <br/><br/>
+> <strong>MRTK v2 (Legacy)</strong>
+> <br/><br/>
+> The old MRTK2 repository, <a href="https://github.com/microsoft/MixedRealityToolkit-Unity">https://github.com/microsoft/MixedRealityToolkit-Unity</a>,
+> is remaining under Microsoft's management and will stay on version 2. Microsoft is committed to the next version of MRTK, and recommends applications
+> move to MRTK version 3 or later. However, Microsoft will continue to support and address critical MRTK2 issues, until MRTK2 is deprecated. Please open
+> MRTK2 issues and discussion topics using the <a href="https://github.com/microsoft/MixedRealityToolkit-Unity">old repository</a>.
+
+<br/><br/>
+
 ![Mixed Reality Toolkit](https://user-images.githubusercontent.com/13754172/122838732-89ea3400-d2ab-11eb-8c79-32dd84944989.png)
 
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