docs: note fork origin

Author: piotr-roslaniec

README.adoc

@@ -4,7 +4,8 @@ https://github.com/keep-network/keep-common/actions/workflows/client.yml[image:h
 https://docs.threshold.network[image:https://img.shields.io/badge/docs-website-green.svg[Docs]]
 https://discord.gg/threshold[image:https://img.shields.io/badge/chat-Discord-5865f2.svg[Chat with us on Discord]]
 
-Common libraries and tools used across Keep repositories.
+Common libraries and tools used across Keep repositories. This repository is a
+fork of https://github.com/keep-core/keep-common[`keep-core/keep-common`].
 
 == Directory structure