Add a release note for chronik

Summary: As per title.

Test Plan: Read it.

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15881

Author: Fabcien

doc/release-notes.md

@@ -4,6 +4,19 @@ Bitcoin ABC version 0.29.0 is now available from:
 
   <https://download.bitcoinabc.org/0.29.0/>
 
+Chronik
+-------
+
+The Chronik indexer is now available as an opt-in option to the Bitcoin ABC
+node software for Linux and Windows. Chronik gives you access to a brand new API
+to get notified of finalized blocks, retrieve transaction history by eCash
+address, gather eToken transaction data, and much more. To enable Chronik,
+simply turn it on with the `-chronik` option.
+
+Take a look at the full [setup and API documentation](https://docs.chronik.xyz/)
+to get an overview of the features, and start building your own application with
+the [chronik-client](https://www.npmjs.com/package/chronik-client) npm package.
+
 Network upgrade
 ---------------