[chronik-client] Add line breaks to README changelog

Summary: The changelog is not being rendered with line breaks in between versions. Add them.

Test Plan: Copy paste to https://markdownlivepreview.com/

Reviewers: #bitcoin_abc, Fabien, tobias_ruck

Reviewed By: #bitcoin_abc, Fabien, tobias_ruck

Subscribers: tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D15664