Bumps @version to 0.4.0 and assembles the four unreleased
fragments into a 0.4.0 section, dated today, per the
changelog.d README. The fragments are deleted in this same
commit. Every bullet carries over verbatim; nothing above or
below the new section moves.

The section leads with a bold Breaking heading carrying the
cache max_messages default change and its what-to-do
sentence, then Added and Changed in the README's order. This
is the first section to honour the pre-1.0 banner's promise
that every breaking change is recorded under a bold Breaking
heading.

Moves the install pin in the README and the getting-started
guide to the version this prepares, so a reader who copies
the snippet gets the release the changelog describes.

The tag and the Hex publish are not here: this commit stops
at the bump and the promotion.

Refs: enc-jup