Add a release note for 0.33.6

Summary: Add a release note for the potential Chronik crash bugfix. It's likely unreachable outside of tests, but better let the Chronik operators know so they can update anyway.

Test Plan: Read it.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D20114