Update mkdocs homepage to have a nice admonition

docs/index.md

@@ -11,4 +11,9 @@ description: Nextflow plugin for sample sheet validation
 
 ==}
 
---8<-- "README.md:6:"
+!!! info
+    nf-schema is the new version of the now deprecated [nf-validation](https://github.com/nextflow-io/nf-validation). Please follow the [migration guide](https://nextflow-io.github.io/nf-schema/latest/migration_guide/) to migrate your code to this new version.
+
+## Introduction
+
+--8<-- "README.md:11:"