Add release notes for Chronik's LOKAD ID index

Summary: Release notes for the new LOKAD ID index. Since it's enabled by default, it's important to highlight how users can avoid reindexing it for hours.

Test Plan: Copy into https://markdownlivepreview.com/

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16086