chore: add contribution notice (#363)

README.md

@@ -1,6 +1,12 @@
 # Aiven client
 [![Go Report Card](https://goreportcard.com/badge/github.com/aiven/aiven-go-client)](https://goreportcard.com/report/github.com/aiven/aiven-go-client) [![Documentation](https://godoc.org/github.com/aiven/aiven-go-client?status.svg)](https://godoc.org/github.com/aiven/aiven-go-client) [![Release](https://img.shields.io/github/release/aiven/aiven-go-client.svg?style=flat-square)](https://github.com/aiven/aiven-go-client/releases)  
 
+## ⚠️ Contribution and Deprecation Notice
+
+This client will eventually be replaced by [go-client-codegen](https://github.com/aiven/go-client-codegen).
+We kindly ask you to open an issue before making any contributions to this repository. 
+
+## About
 
 Aiven is a next-generation managed cloud services platform with a focus on
 ease of adoption, high fault resilience, customer peace of mind and advanced