Add a download-and-run path for non-developers: per-OS install from the
Releases page (desktop app vs CLI binary, with the Gatekeeper / SmartScreen
one-time override each needs), a step-by-step getting-started with the
IMAP/SMTP connection table and Thunderbird / Apple Mail notes, and an
unofficial-&-unsigned disclaimer up top. Reframe the old cargo-centric
sections as 'Build from source', and document that body search covers
downloaded messages while metadata search covers the whole mailbox.
TutaBridge links Tuta's Rust SDK, which is part of the GPLv3-licensed
tutanota project, so the bridge must carry the same license. Add the
full GPLv3 text, set `license = "GPL-3.0-or-later"` on all three
crates, and note it in the README.
Anthony