Deprecate legacy plugin index in README

Updated README to indicate deprecation of legacy plugin index.

Author: ewels

README.md

@@ -1,5 +1,9 @@
 # Nextflow Plugins index
 
+> [!WARNING]
+> This legacy plugin index is deprecated in favor of the new [Nextflow plugin registry](https://registry.nextflow.io/). Starting with Nextflow 25.10, the plugin registry is the _only_ method for downloading plugins, replacing the legacy plugin index.
+> Plugins that require Nextflow >= 25.10 will not be published in this GitHub repository.
+
 This repository stores the index of plugins that can be used by 
 [Nextflow](https://github.com/nextflow-io/nextflow).
 
@@ -26,4 +30,4 @@ which is organised with the following structure:
 The top-most JSON array lists all available plugins. The `releases` array
 lists all the available versions for the corresponding plugin.
 
-Learn more about Nextflow plugins in the [Nextflow documentation](https://www.nextflow.io/docs/latest/plugins.html).
+Learn more about Nextflow plugins in the [Nextflow documentation](https://www.nextflow.io/docs/latest/plugins.html).