[chronik-client] Add example for chronik.token call

Summary:
T3217

I've just been using chronik.tx for this, wasn't aware this was available

Test Plan: Confirm README is accurate and that monorepo version of chronik-client is not behind published version

Reviewers: #bitcoin_abc, emack, tobias_ruck

Reviewed By: #bitcoin_abc, emack, tobias_ruck

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D14223