Explain archival state (enketo#593)

README.md

@@ -1,5 +1,9 @@
 # Enketo Express
 
+### ⚠️ Enketo Express code and issues have moved to [the Enketo monorepo](https://github.com/enketo/enketo) ⚠️
+
+---
+
 _The [Enketo Smart Paper](https://enketo.org) web application._ It can be used directly by form servers or used as inspiration for building applications that wrap [Enketo Core](https://github.com/enketo/enketo-core). See [this diagram](https://enketo.org/develop/) for a summary of how the different Enketo components are related.
 
 **To get started visit our [technical documentation](https://enketo.github.io/enketo-express).**