docs: add note about migration to the readme (#126)

README.md

@@ -5,6 +5,8 @@
 
 Dash Core Documentation
 
+**Note: this documentation was migrated to the [dashpay/docs repository](https://github.com/dashpay/docs) after the Dash Core v21 release. Once Dash Core v22 is released, this documentation will be available at https://docs.dash.org/en/stable/docs/core and no further documentation updates will be made in this repository.**
+
 Welcome to the Dash Core developer documentation. You’ll find sections for
 reference information, API details, guides, examples and Dash Core wallet
 information to help you start working with Dash as quickly as possible. This