Replaces the upstream Notesium README with librenotes-specific
content: project description, multi-tenant goals, build/run
instructions referencing cmd/librenotes, Nix-based dev setup,
fork attribution, and MIT license note.
CI badge points at the workflow that #5 will create. Module path
and directory layout match the structure landed in the previous
fork commit.
Closes#37.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
